12. Not until every single reporter had gone _____ outside his front door again.
(A) did Alan step
(B) when Alan stepped
(C) stepped Alan
(D) Alan did step

Not until every single reporter had gone did Alan step outside his front door again.

When a sentence begins with a "negative adverbial" such as 'in no way', 'at no time', 'not until', 'not often', etc, the subject and the auxiliary verb are inverted. That also happens with adverbs like 'seldom', rarely, 'never'. With those, you actually have to add the auxiliary before the subject of the main clause in the present simple and past simple tenses, (it normally would not be present in "non-inverted" sentences in those tenses).