C. "Thank you, I'm a hunter, not a murderer."
Rainsford is showing here that while he considers himself a hunter. He will not hunt humans. He sees the killing of humans for sport as an act of murder. Therefore, if he was to hunt a person, he would be a murderer. Option A shows Zaroff's disappointment in Rainsford but not why. Option B only shows Zaroff's view of hunting humans as a thrill. Option D is also Zaroff trying to show Rainsford that he is wrong.