Explanation
The expression ‘about to’ is used before another verb to mean that someone was on the verge of doing something. In the given sentence, the person was going to almost finish what he was doing, but unfortunately, the bell rang before that. Hence ‘about’ is the apt answer in the given context. Thus option C is correct. On the other hand, the prepositions ‘against’, ‘above’ and ‘at’ are used to denote a place/position. They are not used along with the preposition ‘to’ to form an expression. These prepositions cannot be used alongside ‘to' either since it would be grammatically incorrect, therefore options A, B and D are incorrect.
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