I think the answer is: B - when he sees what you have done.
Explanation:
Have done --- Have done is a present perfect tense, generally it is used when the action is completed recently/just now.
Had done-- Had done is a past perfect tense, generally refers to something which happened earlier in the past, before another action also occured in the past.
The correct answer is B, forget, because after did, you should always use the bare infinitive (to forget is the full infinitive, bare infinitive is just 'forget', omit 'to).
