Fill in the blank with the most appropriate word.
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 D)before is correct as it indicates a point of time earlier than a certain point of time. The other options are wrong as since, for, and about indicate a duration with its starting time given, a purpose, and a piece of information respectively. These uses don't fit in the given question. The correct answer is D)before.