Question

mr. sen is considered a great authority................grammar.(fill with prepositions)

Answer 1

mr. Sen is considered a great authority in grammar.

Answer 2

Answer:

on

Explanation:

Generally, when someone is an authority, the meaning is that they are an expert, and the preposition "on" is best. The abstract noun "authority", as a thing that you can have, uses the preposition "over." In that case, as you observed, "authority" carries the sense of control or responsibility, rather than knowledge.