Explanation
When we walk barefoot on the sand, the sand yields under our weight. When we push off some of the ground that were meant to support ourselves, gets pushed away rather than staying in place. So, we do not get proper normal reaction i.e. friction force that helps us to walk. [Option D]
$$\textbf{Correct option: Option (D).}$$
$$\textbf{Explanation:}$$
$$\bullet$$Gravitational force is a universal force of attraction between matters due to the virtue of their masses.
$$\bullet$$It acts all the time on all the matters. It is a non-contact force and acts from distance and therefore its formula is given by,
$$F=\dfrac{GMm}{{{R}^{2}}}$$
$$\textbf{Thus, option (B) and (C) both are correct and hence option (D) is correct.}$$
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