Fill in the blank with a suitable preposition.
I will return the library book _______ the afternoon.
The given sentence should be filled with a preposition of time since it speaks about ‘the afternoon’. Thus option D is incorrect, as ‘of’ is not a preposition of time. The preposition ‘on’ is used to indicate a day or date, while ‘at’ is generally used while speaking about a specific time. ‘The afternoon’ doesn’t come under both of these categories, it doesn’t specify a day/date or gives an exact time at which the action takes place. Hence options A and C are incorrect. Preposition ‘in’ is used to speak about a part of a day which is apt in the given context. Thus option B is correct, since it speaks about a part of the day, i.e. the afternoon.