Which term means so subtle?

• A. changing rapidly and often
• B. to swallow
• C. within the body
• D. so subtle

Answer: (D)

Understanding how intensity words work helps here. Subtle describes something delicate, hard to notice, or with fine distinctions. When you attach the intensifier so, you’re signaling an extreme degree of that delicacy or nuance. So, the term that directly conveys “so subtle” is the phrase itself that includes the intensifier, because it precisely communicates an extremely subtle quality. The other options point to unrelated ideas—rapid change, swallowing, or being located inside the body—so they don’t fit the sense of extreme subtlety.