Answer:
The sentence that reflects the central idea of the passage is It may be tasty, but junk food can be harmful to a person's health.
Explanation:
The central or main idea is the most important thought of a paragraph that tells the reader what the text is about. In this passage, the sentence that reflects the central idea is the second one -it may be tasty, but junk food can be harmful to a person's health- because the whole paragraph is aimed at developing that idea.