Since a native speaker has already piped in, I am not a native speaker so I only beg for an answer to clarify my own confusion.
My initial thought to this question is A - with the following logic.
To me, the sentence is comparing two groups of people: modern people (people who still continue to ...) and "ancient" (for lack of a better word) people. So I thought, the structure of the sentence should be
(a) there are people (now) who still continue to make cheese (in the same way as)
(b) there WERE people (in the past) who HAD made cheese for several hundred years (from one time point in the past to another time point ALSO in the past)
thus I would choose A.
If the answer is D, the sentence would seem a little odd to me, because the first group (people who continue to make cheese)
is PART OF the second group (who have been making cheese). Then for me, there is NO NEED for such a comparison.
I hope I've explained myself well enough so that someone can understand and respond to my confusion.