Which answer is the sentence fragment?
a. minor-league players can win you over with their determination and energy.
b. it was "dollar hot dog night" at the local minor league baseball game.
c. admission is only ten dollars, which is about the same price as going to a movie.
d. peanuts and popcorn from the concession stand during the fifth inning.

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.

our founding fathers introduced the revolutionary idea that each person's desire to pursue their idea of happiness was not self-indulgence, but a necessary driver of a prosperous society. they created a government to defend that right for everyone.