They're both correct but they mean different things. In A, in means during, so it means what thing(s) do you usually do during this hour. It could refer to a lunch hour or any hour (period) that is set aside for something. In B, at refers to the beginning of the hour (point). So it means something like: What do you begin to do at this hour. I usually eat my lunch and read my book in this hour. I usually go to bed at this hour.

The “at” one🙂

Both prepositions are correct. Many prepositions would be correct. In addition to "in" and "at" you could also use: "during", "before", "after", "within", "around", "with", "near", "on", "until", "over", "through". Each preposition has its own meaning. Use the one that has the meaning you want to express. "In" connotes containment in the hour. "At" connotes a location in time.

B. "What do you usually do at this hour?"

Hello Thestorea, the correct answer is B. To talk about a specific time, we use the preposition "at". For example, "at five o'clock" or "at this hour". "In" is used for parts of the day, months or years. For example, "in the morning", "in July", or "in 2022".