Which statement best defines 'topic sentence'?

A topic sentence is essential because it serves as the main idea of a paragraph, guiding the reader on what to expect in the upcoming sentences. It succinctly expresses the central theme that the paragraph will expand upon, making it easier for the reader to follow the author’s argument or narrative. By clearly stating this main idea, the topic sentence helps to structure the paragraph and provides coherence to the overall writing.

In contrast to this, other options describe different elements of writing. Supporting sentences are designed to bolster the argument or premise established in a thesis, while concluding statements summarize or wrap up discussions. Rhetorical questions, although effective for engaging readers, do not convey the main idea directly and are not considered topic sentences. Overall, the best definition is indeed the one that identifies the topic sentence as expressing the main idea of its paragraph.