A preposition is a word governing, and usually preceding, a noun or pronoun and expressing a relation to another word or element in the clause.
Because it expresses a relationship, using the wrong preposition can completely change the intended meaning of the clause.
There is a man on the platform.
There is a man beside the platform.
She talked after lunch.
She talked through lunch.
What did you do it for?
What did you do it with?
When asking questions about prepositions, and especially which preposition to use, the intended meaning must be clear in the question. The correct preposition is the one which delivers the intended meaning, so the question must make clear what the intended meaning is.