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) am thinking of changing is correct as of indicates a purpose. The other options are wrong as the given sentence does require a preposition, to indicates direction, and the preposition is missing .These uses don't fit in the given question. The correct answer is B) thinking of changing.