By tradition, when does the presidential campaign begin?

Presidential campaign formally opens when a candidate is officially nominated at the National convention. Therefor your answer would be A.When a candidate is officially nominated at the national convention. ;)

Learned it in school ;)

Presidential campaign formally opens when a candidate is officially nominated at the National convention.

C. In September, following the national convention

All candidates begin the campaign process. By tradition, the national conventions are held in the summer, and the official campaign begins in September. In fact, though, candidates begin to seek support and money long before the national convention.

otherwise i have took the exam

Definitely A. Before that, during the primaries, you're still running against the other hopefuls, even if you have a large lead.

I'd say C, if A weren't up there, but it would only apply for the Republican Party, since they tend to have their convention in September, while the Democrats usually have it in (or near) August.

From observation in the past 20-30 years it seems that the exploration and initial puffery/thumbing of drums begins about 6 years ahead of an actual election.

I think it's the day after Inauguration Day :-(