The only correct answer is D. BOTH OF THESE ARE CORRECT: Excuse me, but there is something which I must speak to you about immediately. Excuse me, but there is something that I must speak to you about immediately. BUT when you invert the subordinate clause, only "about which" is acceptable, "about that" is not a valid construction.

Hi! Just that, "What" is general: Susan: What language do you speak? (Of all Languages) John: I speak three Languages. Susan: Which one (of the three you speak) you speak the best? Actually, that's a good way to grasp the difference, of course, that's far from being exhaustive or illustrative, hope you get it.