Fill in the blank with the most appropriate word.
_____ the afternoon we went for a walk.
A preposition is a word which comes before a noun or a pronoun and establishes a relationship between the elements of a clause or words. Prepositions can come before verbs in the ground form.
There are six kinds of prepositions: Prepositions for Time, Prepositions for Place, Prepositions for Direction, Prepositions for Agent, Preposition for Instruments and Prepositional Phrases.
Option D is correct as afternoon indicates the use of in.
The other options are wrong as afternoon suggests that we are talking a particular time period int he course of a day.
The correct answer is D)in