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 A)at is correct as it is used to indicate an exact location. The other options are wrong as with is used to indicate inclusion, on is used to indicate a surface, above is used to indicate a higher position and over is used to indicate a completed task. These uses don't fit in the given question. The correct answer is A)at.