quite an interesting way to teach topic sentences, because the answer to this question is the best definition of a topic sentence that one can present, it is as if answering the questions taught you how to do it, this is some real teaching, it is terribly well-done.
the topic sentence in this paragraph is:
topic sentences writers stay on track and keep a paragraph from becoming an aimless collection of sentences about an idea.
which incidentally tells you what a topic sentence should be.
what is the context here? we need to know what story you're talking about : )
can you type everything out? we cant see clearly