Can a topic sentence be a question?

Hi,
Do you mean the Heading of a Topic? You can use a question as Topic heading.

The below link will give you more informaton on this.

Writing Reader-Friendly Documents: Use useful headings

Of course.

When crafting an essay, the topic may be an assertion or a question. Which is appropriate depends entirely on your writing style, and your personal appreciation of the subject. In general, if you have an opinion to start with, assertion is more appropriate. If you have no opinion or hypothesis, the question is more likely the correct approach. Starting from an assertion is the classical style, and you're likely more than familiar with it, so I'll focus on the question aspect here.

When approaching a topic with a question, it is very important to avoid any bias in the early stages. You have to focus on each point equally. If you are working towards a predetermined outcome, you have to hide that... otherwise, using the question becomes arrogant instead. Work at the reasonable outcome from each perspective, and you'll end up with a rational explanation for all cases. From there, you have to reconcile the reasonable outcomes with your expected outcome.

Now, if you have no hypothesis, you can avoid this... although that ignores half of the scientific method. :)