The answer is A. relative.
The sentence use the pronoun "that" which can be either a relative or a demonstrative. By analysing the sentence, we can see the pronoun was used to connect (relative) instead of to point at things (demonstrative).
Textual evidence from “The Interlopers” that supports the conclusion that hatred feeds on itself is:
3. "The feud might, perhaps, have died down or been compromised if the personal ill-will of the two men had not stood in the way; as boys, they had thirsted for one another's blood, as men each prayed that misfortune might fall on the other…"
Saki explains the hatred between the Ulrich and the George in the story. As the result of lightning that struck the huge beech tree, both are trapped under the branch of the tree where they are unable to touch each other but can speak. in such heedless situation, Ulrich offers the flask to George, to which he refuses stating he doesn't drink with his enemies. Later Ulrich proposes the treaty of peace and to end this feud. George thinks upon this treaty and accepts the proposal later realizing this feud to be a foolish act. Action falls when they both scream out of help and they see something or someone approaching down the hill. As a result, as they screamed for help, their death arrived down the hill, Wolves.
Is that the entire passage or is that the highlighted portion?