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 B)for is correct as it indicates a specific duration. The other options are wrong as since, in, and at indicate a duration which has its starting point given, inside something, and a specific location respectively. These uses don't fit in the given question. The correct answer is B)for.