A question sentence is easy to identify because it will end with a question mark "?". The sentence will also ask something, it won't tell you something.

Of the choices, only one sentence ends with a question mark and is the correct answer choice, "Did you answer the door?"

If we don't know the answer off the top of our head, let's say the alphabet to help us remember.

A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z

The question asks us which letter comes after M, which we can see is N.