" For his father was as strong as an ox and as stubborn as a mule." is an example of direct characterization
The topic sentence is, of course, a sentence where you state the main topic of your paper. You want people to know your topic as soon as possible. Therefore, answers A and D are obviously places you would most commonly put your topic sentence. However, you can also put it in your body of your paper. But the one place you never want to put your topic sentence is answer C, in your conclusion paragraph.
I think the answer is either
A. Why does Harry seem sad at the end of the passage.
or
D. Where will jack and Harry find work after the professor leaves?