Who ___ done this work?

The correct auxiliary verb to complete the sentence "Who ___ done this work?" is "has". This is because "done" is the past participle of "do", and when forming the present perfect tense with a singular subject like "Who", the auxiliary verb "has" is used. The sentence structure requires a verb that forms the present perfect tense with a third-person singular subject. Therefore, the complete and grammatically correct sentence is "Who has done this work?"