The correct option is C with
Option C (with) is the correct answer. In this sentence, one person is agreeing or accepting someone else's idea or point of view. Hence, we use the word 'with'. Option A (upon) is incorrect because we use the phrase 'agreed upon' to express that something is noncontroversial. Option B (to) is incorrect because the phrase 'agree to' is followed by a thing/concept and not a person.