Opener shouldn't have 3S and a singleton diamond w/o either raising 1S to 2S or showing belated support (extra values; only 3S) if not passed in 2C, so you don't need to re-bid 2S to avoid missing a 9 card fit. Opener can have substantial extra values ...
I'd double. If lefty ran to 5D, I wouldn't defend unless partner hit it, but if he ran to 4NT, and RHO bid 5D, I'd pass ("defend"?), hoping partner would expect this sort of pointed disparity, and realize he had to choose between DBL & 5H. I'm ...
I re-read the question. My emotional response is closest to C). I wouldn't have been the one to SAY that, but it is the answer at which I smiled. I have to follow the rules, but if the danger of cheating or damaging someone's concentration is SO great ...