We would like to enlist your help in arbitrating this grammatical dilemma.
Given the question:
What does the door do?
Which of the following options is most correct as a response to the question?
Please pick only from the given options and do not modify them in any way. (I have asked people in the past and they have responded with more-than-one-word answers. )
Please explain your answer. There are 20 dollars riding on the answer to this question.
They are both correct: they elide the beginnings of different responses.
What does the door do? [What the door does is] close.
What does the door do? [The door] closes.
If you look at the verb be, you find that the former phrasing seems to be more productive and natural, if not necessarily more correct. Using the infinitive mirrors the structure of the question.
What does the Pope do? [What the Pope does is] be Catholic.
What does the Pope do? [The Pope] is Catholic.
I asked a question about this once that may be interesting.