So why ________ we need to sleep?

• A. do
• B. could
• C. should
• D. would

Answer: (A)

Forming present simple questions with a wh-word requires an auxiliary like do. When the main verb is in its base form, we add do (or does for third-person singular) before the subject to make the question. So the natural order is wh-word + do/does + subject + base verb + rest of the sentence: Why do we need to sleep?

This keeps the tense and truth of the statement intact while turning it into a question. The other options would change the meaning: could would ask about possibility, should would imply obligation or recommendation, and would would sound hypothetical.