C. A topic sentence is the central idea of a paragraph, while a main idea is the central idea of an entire text.
Explanation:
Remember that in a paragraph the most important sentence is the topic sentence. It's function is to organize the information presented in it; it summarizes that information. It is usually the first sentence found in a paragraph and it is followed by sentences that give more details.
The main idea can refer to the one found in a paragraph or the one found in a text. It is important to say that it can be expressed in a direct way (for example a sentence), or it can be implied (the reader can find it by reading the text). It makes reference to the topic of the passage, what it is about. It answers: Who or what is being discussed?
The best answer choice for the definition of a topic sentence opposed to a main idea is going to be: C. A topic sentence is the central idea of a paragraph, while a main idea is the central idea of an entire text. I hope that answers your question.
