Answer:
It's a sentence that states the main idea of the paragraph
Explanation:
ex: There are many reasons why pollution in ABC Town is the worst in the world. The topic is "pollution in ABC Town is the worst in the world"
https://examples.yourdictionary.com/examples-of-topic-sentences.html
It's the very first sentence at the very beginning telling you the main idea of the paragraph on what you will be reading about.
