So why ________ we need to sleep?

• A. are
• B. do
• C. will
• D. could

Answer: (B)

When forming a question in the present simple with a main verb like need, you use do-support. Since there’s no auxiliary verb already, you add do, then invert the subject and the auxiliary: do we. The verb that follows stays in its base form, and the to-infinitive remains: to sleep. So the natural, correct question is: So why do we need to sleep?

Other options don’t fit because they would either use a different auxiliary that isn’t appropriate here (are would imply a form of be, as in Are we sleeping?), or would change the meaning to future or possibility (will or could). The presented sentence is asking about the general necessity in the present, which is exactly what do + we + need + to sleep expresses.