The excerpt that best helps readers to visualize how thin Buck has become is:
"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 description vividly illustrates Buck's emaciated condition, emphasizing the sharp outline of his bones and the loss of flesh, which effectively conveys his starvation and physical deterioration.