Usher believes that one of the main causes of his poor health is that he is sad because his sister, who is the only living relative he has, is sick and may be close to death, as shown in option A.
We can arrive at this answer because:
This question is about "The Fall of the House of Usher" which tells the story of a house that was dark and decaying as well as its owner's mental state.
You can find more information about this short story here:
https://brainly.com/question/11438422