Motorcyclists can make themselves more visible by
a. driving at slower speeds
b. driving in the center of the lane
c. putting reflective tape on their helmet and clothing
d. using high beam most of the time

where exactly is the sentence

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.