D is correct as a topic sentence is a sentence that expresses the main idea of the paragraph in which it occurs. It also helps the reader to understand the writer's ideas better as it guides the reader as to the interpret the main idea, because of the topic sentence leading them to it.
B is incorrect as the topic sentence has a larger function than just mentioning the main topic of the paragraph. As both A and C are correct, hence, only A and C are incorrect.