So the correct option is D.

• Exotic species - They are those species which are moved by humans from one place to another or are accidentally carried away. They are also termed as alien or invasive species.
  
• Pioneer species - They are the first ones to be present in the succession and are hard algae or moss. They provide stability to the ecosystem.
• Edge species - They are the species of their own kind, if they got extinct then they cannot be replaced by any other species. EDGE refers to Evolutionarily Distinct and Globally Endangered species.
• Keystone species - They are species that have the main role in the maintenance of an ecosystem, they support the community structure and ecosystem even at the time of food scarcity. If they are not present then it will lead to the destruction of an ecosystem.

So, the correct option is 'Keystone species'.

The primary producers convert only $$1\%$$ of the energy in the sunlight available to them into NPP. Out of this, only $$10\%$$ is available for the organisms at the next trophic level.

Primary succession is ecological succession that begins in essentially lifeless areas, such as regions in which there is no soil or where the soil is incapable of sustaining life. So, it starts in an area where no living organisms ever excited.

The affinity of hemoglobin is approximately 200 times more than respiratory oxygen (O$$_2$$). A small concentration in the atmosphere could also cause toxicity and be able to affect health.

In a terrestrial ecosystem, the green plants capture about 1% of the energy from the sunlight that falls on their leaves and converts it into chemical energy.

In a sea or ocean ecosystem the biomass of producers namely, the diatoms, the dinoflagellates and other phytoplanktons is lower than the herbivorous fishes like surgeonfish and other herbivores  which is again lower than  the carnivorous fishes like Shark etc.

• Pioneer species are those species which gets colonize in the area which did not contain any inhabitant previously. Algae are the best example of pioneer succession as it invades the barren rocks. In general, pioneer species are autotrophs or those that maintain harmonious ecological interaction with autotrophic bacteria, herbaceous plants, lichens.
• Autogenic succession is succession driven by the biotic components of an ecosystem. 
• In contrast, allogenic succession is driven by the abiotic components of the ecosystem. 
• Establishment (ecology) or ecesis is the process by which a plant or animal becomes established in a new habitat. 

Gyps is a genus of vultures characterized by bald head, broad wings and mainly dark plumage. They are large scavenging birds, feeding mostly from carcasses of dead animals and thereby forming an important link in the decomposes food chain.
Thus, the correct answer is (B) Gyps.

The inorganic nutrients cycle though more than the organisms, however, they also enter into the atmosphere, the oceans and even rocks. Since these chemicals cycle through both the biological and the geological world, we call the overall cycles biogeochemical cycles. 

The large annual input of sulfur from burning of coal and other fossil fuels adds a substantial amount SO$$_2$$, which acts as an air pollutant. Hence, the presence of sulfur in any area indicates the high level of coal or fossil fuel burning there. 

Edaphic nutrient cycle relates to the cycle, which involves the soil. The edaphic nutrient cycle involves several biogeochemical cycles like carbon cycle, phosphorus cycle, oxygen cycle, nitrogen cycle and sulphur cycle. A nutrient cycle is the movement and exchange of organic and inorganic matter back into the production of living matter. The process is regulated by food web pathways, that decompose matter into mineral nutrients. Nutrient cycles occur within ecosystems. Ecosystems are interconnected systems, where matter and energy flow and is exchanged as organisms feed digest and migrate about. Minerals and nutrients accumulate in varied densities and uneven configurations across the planet.

The Mesozoic Era is an interval of geological time from about 252 to 66 million years ago. It is also referred to as the age of reptiles. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the Cenozoic era. The atmosphere present in mesozoic era contains nitrogen and carbon atoms. The climate of the Cretaceous is less certain and more widely disputed. Higher levels of carbon dioxide in the atmosphere are thought to have caused the world temperature gradient from north to south to become almost flat: temperatures were about the same across the planet. Average temperatures were also higher than today by about 10$$^o$$C. In fact, by the middle Cretaceous, equatorial ocean waters (perhaps as warm as 20$$^o$$C in the deep ocean) may have been too warm for sea life, and land areas near the equator may have been deserts despite their proximity to water. The circulation of oxygen to the deep ocean may also have been disrupted. For this reason, large volumes of organic matter that was unable to decompose accumulated, eventually being deposited as "black shale".

• Primary productivity is the rate at which energy is converted by photosynthetic and chemosynthetic autotrophs to organic substances. The total amount of productivity in a region or system is gross primary productivity. 
• Secondary productivity is one portion of energy flow in a community, which includes the ingestion and assimilation of energy, and the expenditure of energy in metabolism by consumer organisms.
  

Ecological pyramids were developed by Charles Elton (1927) and are, therefore, also called as Eltonian pyramids. In most ecosystems, all the pyramids, of number or of energy or biomass are upright. The pyramid of number in a tree ecosystem is inverted. The pyramids of grassland and pond are upright.

• In a grassland ecosystem, the number of producers (mainly grasses) is always maximum, followed by decreasing numbers at second trophic level (herbivores), third trophic level (carnivores) and least number of apex predators. 
• Thus, a pyramid of numbers in grassland is erect. 
• Hence, The pyramid of number in a grassland ecosystem is Always erect. 
• So, the correct answer is 'Always erect'.