Which of the Following Would Best Illustrate an Affirmative Defense

Which of the following would best illustrate an affirmative defense?

A) The accused admits to shooting his neighbor, but says it was self-defense.
B) The accused admits his neighbor was shot, but denies he shot him.
C) The accused admits he saw someone shoot his neighbor, but acknowledges he did nothing to stop it.
D) The accused admits to shooting his neighbor and offers no justification for the act.