Not a constituent of air –

**Core Concept**
The question is testing the student's knowledge of the composition of air, which is a mixture of gases essential for human respiration. The correct answer will be a gas that is not present in the air we breathe.

**Why the Correct Answer is Right**
The air we breathe is composed of approximately 78% nitrogen (N2), 21% oxygen (O2), 1% argon (Ar), and trace amounts of other gases. The absence of a particular gas in air indicates that it is not a constituent of air. In this case, the correct answer is a gas that is not present in significant amounts in the atmosphere.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it is a constituent of air.
* **Option B:** This option is incorrect because it is a constituent of air.
* **Option C:** This option is incorrect because it is a constituent of air.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the composition of air can vary depending on factors such as altitude, pollution, and location. However, the primary constituents of air remain relatively constant.

**Correct Answer:** D.