Answer:
3. However, regular exercise and healthy nutrition can certainly help us age better.
Step-by-step explanation:
The topic sentence of a paragraph is its most important sentence. It tells us what the rest of the paragraph will be about, summarizing the information it contains. It's usually its first sentence.
However, in this case, it isn't like that. A question can't be the topic sentence. The topic sentence will be the answer to that question. That answer is present in the third sentence: However, regular exercise and healthy nutrition can certainly help us age better. The rest of the paragraph gives us examples of how exercise and healthy nutrition can help us. This is why the third sentence is the correct answer.