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 C)in is correct as it indicates before or at the end of a particular period. The other options are wrong as at,on, and by indicate a specific location,a surface, and usage respectively. These uses don't fit in the given question. The correct answer is C)in.