Answer:
The best option is letter D. With a docile nature, manatees are often called sea cows because of their appearance.
Step-by-step explanation:
A topic sentence is supposed to inform readers on what the paragraph is about. It lets readers know where the author is going and how they are getting there in terms of the idea being developed. The sentences that come after the topic sentence function as its support, as evidence to defend the topic.
The best topic sentence for section 6 would be "With a docile nature, manatees are often called sea cows because of their appearance." The paragraph in section 6 corroborates the information stated in this topic sentence. The sentences describe what the manatees look like; how much they weigh; how peacefully they spend their days moving slowly, eating (sea) grass - just like a cow. Thus, the sentences in section 6 function as evidence that would support the claim that manatees are docile and that they look like cows.