whats difference between which and that ?
Which one is more suitable D or C?
Speaker 1: “Excuse me, but there is something about _____ immediately.” Speaker 2: “Certainly.”
a. that I must speak to you about
b. which I must speak to you about it
c. that I must speak to you
d. which I must speak to you

Question

whats difference between which and that ?
Which one is more suitable D or C?
Speaker 1: “Excuse me, but there is something about _____ immediately.” Speaker 2: “Certainly.”
a. that I must speak to you about	
 b. which I must speak to you about it	
 c. that I must speak to you	
 d. which I must speak to you

Answer

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.

Answer

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.