Answer:
b. John Thornton.
Explanation:
John Thornton is the one who eventually rescues Buck. Thornton has told the team that the ice is not safe, and that they should not continue their journey. However, Hal decides to ignore him, and continues abusing the dogs. Buck is absolutely exhausted, and as he believes he cannot continue, he gives up. Thornton jumps in before Hal does anything else to the dog, and threatens to kill him if he harms Buck again.
