Which term means 'calm, tranquil'?

• A. respiration
• B. retain
• C. sublingual
• D. serene

Answer: (D)

Understanding vocabulary for describing mood or atmosphere: calm and tranquil are about a peaceful state. Serene specifically means untroubled calm, peaceful, and undisturbed, which makes it the best fit for “calm, tranquil.” The other terms refer to unrelated ideas: respiration is the process of breathing, retain means to hold onto or keep, and sublingual describes something under the tongue (a medical term about administration). Those don’t convey calm, so they don’t fit as well. Serene carries the nuance of quiet composure and peacefulness that matches the given meaning.