Answer :
"B. it came into his mind that he should shoot its and end it pain and he raised the gun" is the best option in terms of explaining the boy's decision concerning the buck.
In this short story, Dorris Lessing describes a fifteen year old boy who goes hunting every morning. One day the boy notices a buck that is wounded. It was about to die. This scene continues in the story with the following sentences: "It came into his mind that he should shoot it and end its pain; and he raised the gun. Then he lowered it again."