The question is about understanding the function of a topic sentence in a paragraph. - The correct answer is C. To state the main idea, as a topic sentence introduces the main point of the paragraph. - Option A, To provide supporting details, is incorrect because topic sentences do not provide details; they introduce the topic. - Option B, To conclude the paragraph, is incorrect because a concluding sentence is used to wrap up the paragraph, not the topic sentence. - Option D, To describe the setting, is incorrect because a topic sentence is not focused on describing a setting but on presenting the main idea.