The correct answer is "focus". A topic sentence usually comes at the beginning of a paragraph. The purpose is to tell what the paragraph is going to focus on. Since a paragraph usually talks about an aspect of the topic being discussed, the topic sentence says which aspect will be focused on. A possible answer could also be 'clarity', but other parts of the paragraph, such as explanations and examples, are also used with the purpose of clarity. Thus, the feature that distiguishes a topic sentence is 'focus'
