As you said, David, both A and B are correct, so if this sentence appears in a test, then the answer should include both, not either A or B, right?
Yes, that's right. Given that both (A) and (B) are correct, neither should be marked as incorrect. Therefore, if both are listed as answer choices on a test, there should be a third choice saying "both (A) and (B)."
I don't think I had noticed Ahmed_btm's post when I made my first post above; we must have been posting at roughly the same time. My answer is not contradicted by the Longman editor's views.
The editor at Longman is NOT saying that the past perfect is incorrect. Rather, the assertion is that the past perfect (backshift) is not needed in the subordinate reported-speech clause. That it is not needed does not imply that it is incorrect.
There are examples where the since-clause would HAVE TO be backshifted:
(1a) John said, "It has been five years since I have eaten Chinese food."
(1b) John said that it had been five years since he had eaten Chinese food.
Without backshift in the since-clause of (1b), (1a) would be misrepresented. "Had eaten" is necessary in (1b). Backshift in the since-clause is optional in your sentence. I have explained why in my first post above.