Final answer:
Carlson insists on shooting Candy's dog because the dog is too old to work, suffers, and smells bad, with Carlson believing it's more merciful to end its life. The correct option is A.
Step-by-step explanation:
Carlson insists on shooting Candy's dog because A) The dog is too old to work. In John Steinbeck's novel Of Mice and Men, Carlson argues that the dog is not only too old and incapable of working, but also that it is suffering and in constant pain.
Additionally, Carlson complains that the dog smells bad, which is another reason he provides for wanting to end the dog's life.
Therefore, Carlson believes that shooting the dog would be the most merciful action, despite Candy's attachment to his long-time companion.