**Core Concept**
The partial pressure of oxygen in the expired air is a measure of the concentration of oxygen in the exhaled breath, which is an indicator of the efficiency of gas exchange in the lungs. This concept is crucial in understanding respiratory physiology and pathophysiology.

**Why the Correct Answer is Right**
The partial pressure of oxygen in the expired air is approximately 10-15 mmHg, which is significantly lower than the inhaled air (approximately 150 mmHg). This is because oxygen is being absorbed into the bloodstream through the alveoli, leaving less oxygen in the expired air. The oxygen is diffusing from the inhaled air into the blood, where it binds to hemoglobin and is transported to the body tissues. The remaining oxygen in the expired air is mainly nitrogen, which does not participate in gas exchange.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the partial pressure of oxygen in the expired air is not around 100 mmHg, which is closer to the inhaled air.
**Option B:** This option is incorrect because the partial pressure of oxygen in the expired air is not around 20-30 mmHg, which would indicate inefficient gas exchange.
**Option C:** This option is incorrect because the partial pressure of oxygen in the expired air is not around 5-10 mmHg, which would indicate severe respiratory failure or anemia.

**Clinical Pearl / High-Yield Fact**
The partial pressure of oxygen in the expired air can be used as a non-invasive indicator of respiratory function, particularly in patients with chronic obstructive pulmonary disease (COPD) or other respiratory conditions.

**Correct Answer: C. 10-15 mmHg**