Which is the best example of a strong topic sentence? School can be very challenging for a student.
Why do students have to take six classes every day?
With six classes a day, middle school is challenging.
Social studies is a class students must take.

With six classes a day, middle school is challenging.

Explanation:

The topic sentence of a paragraph is the sentence that summarizes what the paragraph will be about. It's usually its first and most important sentence.

A strong topic sentence is one that clearly states what the paragraph will be about without being wordy or difficult to understand.

The best example of a strong topic sentence is With six classes a day, middle school is challenging. We can see that most of the sentences mention different classes and students. The third sentence is the only one that mentions middle school, providing information the rest of the sentences don't contain. It's important for the topic sentence to include all necessary information. For example, if we didn't mention middle school in the topic sentence, wouldn't know what students exactly the paragraph is about.

This is why the third option is the correct one.

we need more information in order to answer.