Answer:
The meaning of the underlined word is letter d. skilled or capable.
Step-by-step explanation:
We can safely assume your question refers to the word apt because of the options provided. In different contexts, apt may take on different meanings. In this specific case, it means d. skilled or capable. The sentence "Apt scholar that he was, they were equally apt teachers, never allowing him to linger long in error, and enforcing their teaching with their sharp teeth" refers to Buck and other dogs' capacity to learn and teach. Buck was a fast learner, but the two other dogs on his side were also skilled teachers, punishing him whenever he made a mistake. Another example with apt referring to someone's skill or natural ability would be "We have some apt trainees this year."
"The Call of the Wild" is a short novel by author Jack London. The main character is Buck, a dog that gets stolen from his comfortable home to be sold as a sled dog. Buck quickly adapts to his new circumstances, going from a domestic dog to a wild one, being accepted by wolves in the end.