Which word completes: So why ________ we need to sleep?

• A. are
• B. will
• C. do
• D. could

Answer: (C)

The sentence is in the present simple, and when there’s no auxiliary verb, English uses do-support to form a question. The base statement is “we need to sleep.” To turn that into a question, you insert the auxiliary before the subject: “Why do we need to sleep?” The “So” at the start keeps the idea connected to something said before.

Why this fits best: using do + we + need follows the standard present-simple question structure with the main verb “need.” The other options don’t fit the grammar here—are would require a different construction (like “we are needing” or a passive form), will would query a future need, and could changes the nuance to a question about possibility.