He believes that others that support slavery still understand that their people
Then it’s the manhood of the slaves conceded
The detail that best reveals that Mr. Rainsford opposes Zaroff's idea of the ideal prey is Mr. Rainsford's response near the end of the passage when he states that he is a hunter, not a murderer. This statement shows the reader that Mr. Rainsford believes that Zaroff hunting humans as prey makes him a murderer.
I'd say the fact that the characters are talking animals is a pretty good indicator that this story was made for children.
The answer is B. Inexhaustible