Not a constituent of air –

**Core Concept**
The question is testing the composition of air, which is a mixture of gases essential for respiration. Air is composed of approximately 78% nitrogen, 21% oxygen, 1% carbon dioxide, and trace amounts of other gases.

**Why the Correct Answer is Right**
Oxygen is a critical component of air, making up about 21%. It is essential for cellular respiration, where it is used to generate energy through the process of oxidative phosphorylation. Oxygen binds to hemoglobin in red blood cells and is transported to tissues, where it is used to produce ATP. The high concentration of oxygen in the air allows for efficient gas exchange in the lungs.

**Why Each Wrong Option is Incorrect**
**Option A:** Nitrogen is a major constituent of air, making up about 78%. It is an inert gas that does not participate in respiration.
**Option B:** Carbon dioxide is a minor constituent of air, making up about 1%. While it is not a primary constituent, it is still present in the air and plays a critical role in the respiratory process.
**Option C:** Argon is a trace gas present in the air, making up about 0.93%. It is an inert gas that does not participate in respiration.

**Clinical Pearl / High-Yield Fact**
The composition of air is critical for human respiration, and changes in air composition can have significant effects on human health. For example, a decrease in oxygen levels can lead to hypoxia, while an increase in carbon dioxide levels can lead to hypercapnia.

**Correct Answer:** B. Carbon dioxide is not a constituent of the air; however, the question asks for a gas that is not present in the air.