1) C) weak and gaunt
Buck is described as have muscles which "had wasted away" and it goes on to say that "each rib and every bone...were outlined cleanly through the loose hide..." This description shows that he is weak and gaunt.
2) hearing
Sensory details appeal to the senses. In this excerpt, there are many sounds, such as bursting, throbbing, soft-sighing, snapping, and trickle. These all appeal to the sense of hearing.
3) B) His muscles had wasted away to knotty strings, and the flesh pads had disappeared, so that each rib and every bone in his frame were outlined cleanly through the loose hide...
This is the best answer. Thin is another word for gaunt. The reasons this excerpt is the answer is the same as for number one.
4) A) how furious and upset Thornton is
We can see Thornton's anger when it says "suddenly...John Thornton sprang upon the man". In this line he is being described as verbally attacking the man with the club. He does so because he is furious. His feelings about the situation show when his voice is described as "choking". This choice of words shows that Thornton cares about his son and is upset when he speaks.
