At D's Rape Trial, He Asserted a Defence of Mistaken

At D's rape trial, he asserted a defence of mistaken belief in consent. Which of the following is correct:

A) D will succeed if his belief is genuine.
B) D will succeed if there is evidence that the victim has had several previous sexual partners.
C) A merely honest belief will deny the MR for rape.
D) D's belief needs to be both honest and reasonable.