Buck's owner (s) after Perrault and Francois is (are) ... a. Hal, Mercedes, and Charles. c. the man in the red sweater. b. John Thornton. d. the Scotch half-breed who delivers mail.

The answer is actually letter d. the Scotch half-breed who delivers mail.

Explanation:

The characters mentioned in the question belong to Jack London's novel "The Call of the Wild". Buck is a dog that lives comfortably on a California ranch until he is stolen. Buck is then sold to Perrault and François - two mailmen who deliver dispatches to outposts throughout the frozen North - to become a sled-dog. After their final run, they sell Buck to the Scotch half-breed, who also works the mail and constantly overworks the dogs. It is while belonging to him that Buck finally becomes the lead dog of the team.