What is a 'topic sentence'?

A topic sentence is essential because it encapsulates the main idea of a paragraph, guiding readers on what to expect in the text that follows. This sentence typically appears at the beginning of the paragraph, setting the context for supporting details and arguments that will elaborate on the main idea. By clearly stating the focus of the paragraph, the topic sentence helps to organize the content and improves clarity for the reader.

The other choices do not accurately describe the function of a topic sentence. While summarizing text or introducing a new paragraph involves expressing central thoughts, they extend beyond the specific role a topic sentence plays. Similarly, concluding a paragraph serves a different purpose by wrapping up the ideas presented rather than introducing or summarizing them. This distinction underscores the importance of the topic sentence in effectively communicating a paragraph's primary focus.