After the modal verb 'must', we use the base form of the verb, so 'do' is correct. Modal verbs (must, can, should, will, etc.) are always followed by the base form. Option A 'did' is past tense, incorrect after modals. Option C 'does' is third-person singular present, also wrong. Option D 'done' is a past participle that needs a helper verb, not the base form.