Answer:
The correct answer is C - main subject.
Step-by-step explanation:
In English, the topic of something is what the paper is centered around. For example, if you write an email and add a subject line, you are summarizing it very shortly. If I wrote a huge email explaining the benefits of healthcare, my subject would be "The Benefits of Healthcare." Therefore, C is your answer.
The answer cannot be A because your opinion is what is used to gain the topic of an essay. Your opinion is your stance on the topic and therefore is not a topic but rather your belief in the matter.
The answer cannot be B because the issue is why you would write the paper and does not mean that is what the paper will be about. For example, if my paper was misgraded, I would write an email about how upset I am. However, the topic would be to get the grade changed.
The answer is not D because a thesis statement is a broad opening into the evidence that supports the claim or topic of your essay.
I hope this helps.