They are putting you on the spot to declare why you think you deserve a C or better. Standard teacher trick. If you seem satisfied with a D, then it's obvious to them that you know you didn't deserve or earn anything better.
You always end your reply with "Whatever it takes to prove this to you, I'm willing to do." and have every intention to do it... going in to talk to them or taking an extra test is a small price to pay for a better grade in school and they know this. If you can't be bothered with the effort, you don't deserve the better grade. For most classes, grades in college aren't earned in the classroom, but rather what you accomplish on your own time. I've had classes where everything taught in the lecture wasn't on the test, because that stuff is what you should know as a bare minimum... as a result of just being REGISTERED in the class.
(Now insert a lousy joke about doing something sexual. ha ha.. we get it... some of you haven't quite gotten over how cool junior high was.)