Modal verbs help determine the mood of a sentence, to indicate obligation, permission, possibility and so on. The underlined modal verb "can" indicates ability, "Barry" did not have the ability to do "this" without the listener "you". But 'can' isn't the suitable option here as it's used in the present tense, whereas the action "have done" is in the past. 'Could' is a modal that indicates an ability in the past tense, hence option C is correct and A is wrong. 'May' and 'might' indicate possibility ('might' is used as the past tense of may); the sentence does not imply possibility but ability, hence options B and D are incorrect.