**Core Concept**
The use of **nitrous oxide** for analgesia requires careful consideration of oxygen concentration to avoid hypoxia. **Nitrous oxide** is a widely used anxiolytic and analgesic gas in medical and dental procedures. The key principle is to ensure adequate **oxygenation** while minimizing the risk of hypoxia.

**Why the Correct Answer is Right**
To prevent hypoxia, a minimum concentration of **oxygen** is necessary when using **nitrous oxide**. The correct concentration is at least **20% oxygen**. This ensures that the patient receives sufficient oxygen to maintain adequate **oxygen saturation** and prevent hypoxic complications.

**Why Each Wrong Option is Incorrect**
**Option A:** Insufficient oxygen concentration, increasing the risk of hypoxia.
**Option B:** Still not the minimum required to ensure patient safety.
**Option D:** Exceeds the necessary concentration but is not the minimum.

**Clinical Pearl / High-Yield Fact**
Always remember that when using **nitrous oxide**, at least **20% oxygen** should be used to prevent hypoxia. This is crucial for maintaining patient safety during medical procedures.

**Correct Answer:** D. 20%