Choose the option that fills the blank with the most suitable 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)on is correct as it indicates payment. The other options are wrong as for, at, and in indicate a purpose, a specific location, and the inside of an area respectively. These uses don't fit in the given question. The correct answer is C)on.