Fill in the blank with a suitable preposition:
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. Option C)for is correct as it is used to indicate a purpose. The other options are wrong as at, after, and before indicate a specific location, something following in, and a point of time earlier than another the respectively. These uses don't fit in the given question. The correct answer is C)for.