Answer:
mustn't we
Explanation:
So, basically, question tags are short constructions added to the end of a statement in order to emphasize its interrogative character.
Their formation is usually quite simple; they use opposite form of the statement's auxillary verb and the subject. So, if the statement is positive, question tags will have negative form of the auxillary verb and vice versa; if the statement is negative, question tags will have positive form.
Of course, there are some exceptions, but for this particular example, things are basic:
"We must lock the doors" is a statement, "must" is the auxillary verb in positive form and "we" is the subject.
So, to form a tag question, we use opposite form of the verb, in this case "mustn't" and form a question with the same subject "we".
