**Core Concept**
Residual volume is the amount of air left in the lungs after a maximal expiration. It is an essential component of lung function, as it represents the volume of air that remains in the lungs even after a person has exhaled as much as possible.

**Why the Correct Answer is Right**
When a person exhales maximally, they are able to expel most of the air from their lungs. However, some air remains trapped in the lungs due to the elastic recoil of the lung tissue and the resistance of the airways. This trapped air is known as residual volume. The residual volume is approximately 1,200 mL in a healthy adult. It is an important factor in determining lung function and capacity.

**Why Each Wrong Option is Incorrect**
**Option A:** Maximal inspiration is the process of taking the deepest breath possible, which would actually increase lung volume, not decrease it. Therefore, it is not related to residual volume.

**Option C:** Normal inspiration is the process of taking a regular breath, which would not result in the expulsion of air from the lungs. Instead, it would result in the inhalation of air into the lungs.

**Option D:** Normal expiration is the process of exhaling regularly, which would result in the expulsion of air from the lungs, but not necessarily to the point of maximal expiration.

**Clinical Pearl / High-Yield Fact**
Residual volume is an important factor in determining lung function and capacity. It can be affected by various conditions, such as chronic obstructive pulmonary disease (COPD), which can cause an increase in residual volume due to airway obstruction.

**✓ Correct Answer: B. Maximal expiration. This is the volume of air in lung after maximal expiration.**