Did you agree ______ Ginny’s plan ______ the Halloween party?
My answer to this is C, but my colleague thinks it should be B.
It comes down to the difference between 'agree to' and 'agree with'
agree with; approve of (something) with regard to its moral correctness : I'm not sure I agree with abortion.
agree to, or to do something: consent to do something that has been suggested by another person
Hence, you are correct.