The correct answer is A. Leonardo was fascinated with human anatomy
Step-by-step explanation:
In writing, a topic sentence refers to the most important sentence in a paragraph, because this sentence that summarizes all the ideas of a paragraph or the sentence that expresses the point a paragraph is trying to explain or develop. This idea can be identified because it is usually the first sentence in the paragraph or is placed at the beginning of it and because this sentence encloses all the content presented.
Considering this, in the paragraph presented the topic sentence is "Leonardo was fascinated with human anatomy" not only because this is the first sentence, but because it summarizes the content of the paragraph considering other sentence such as "He conducted medical research in hospitals in Florence and Rome" or "For example, he made detailed drawings of a human fetus and a human heart" explain this topic sentence. Therefore, "Leonardo was fascinated with human anatomy" enclosures the rest of the sentences in the paragraph and therefore is the topic sentence.