**Core Concept**
The residual volume (RV) of the lungs is the amount of air left in the lungs after a maximal exhalation. It represents the volume of air that cannot be expelled from the lungs, even with forced expiration. RV is an important component of lung function and is essential for maintaining normal blood gas levels.

**Why the Correct Answer is Right**
The correct answer, 1.2 L, is based on the average residual volume of an adult male, as measured by spirometry or other lung function tests. The residual volume is maintained by the elastic recoil of the lung tissue and the negative intrathoracic pressure, which prevents air from escaping from the lungs during exhalation. The diaphragm and other accessory muscles also help to maintain the residual volume by contracting and increasing the intrathoracic pressure.

**Why Each Wrong Option is Incorrect**
**Option A:** 3.0 L is an excessively high value for residual volume, as it would be difficult to maintain such a large volume of air in the lungs after maximal exhalation.
**Option B:** 0.9 L is too low, as it would not provide adequate lung function and would likely lead to hypoxemia and other respiratory problems.
**Option D:** 1.9 L is also incorrect, as it is higher than the average residual volume and would not be consistent with normal lung function.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that residual volume is an important indicator of lung function and can be affected by various respiratory diseases, such as chronic obstructive pulmonary disease (COPD) and pulmonary fibrosis. A reduced residual volume can indicate lung over-inflation, while an increased residual volume can indicate lung under-inflation.

**✓ Correct Answer: C. 1.2 L**