which homophone best completes the sentence? not going to believe what i just heard about the principal…

Brief Explanations:

The sentence needs a subject - verb combination. "You're" is a contraction of "you are" which fits the context as it implies "you are not going to believe...". "Your" is a possessive pronoun and doesn't fit. "Its" is a possessive form of "it" and "It's" is a contraction of "it is" which are not relevant here.

Answer:

B. You're