The correct answer is option letter D (While there are many different ways to study, the most effective approach is to eliminate anything that might keep you from concentrating on the material.). The sentence presented in option D is the one that best represents the main idea of the essay about the different approaches to studying. Option D could be the thesis statement of the essay, since it states not only the main topic required in the essay question (different approaches to study) but also one approach as the most effective, which could be further developed in the body paragraphs. Option A is incorrect because it does not mention the main topic required in the essay question; it mentions “study habits” instead of “study approaches”. Option B is incorrect because it presents just one point that could be part of the essay and not the main idea of the essay. Option C is incorrect, since it presents just an example to be used in a body paragraph and not the main idea of the essay.