Fill in the blanks with the most appropriate words.
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)by/on is correct as by and on indicates an agent and an occasion respectively. The other options are wrong as of, off, and in indicate possession,removal of something, and inside something respectively. These uses don't fit in the given question. The correct answer is B)by/ on.