**Core Concept**
Normal expiration is a vital process that involves the coordinated effort of the respiratory muscles, lung parenchyma, and airways to expel air from the lungs. The end of normal expiration represents the point at which the lungs have reached their residual volume (RV), and no further air can be expelled without the initiation of forced expiration.

**Why the Correct Answer is Right**
The end of normal expiration is characterized by the closure of the glottis (the opening between the vocal cords) and the relaxation of the diaphragm and other respiratory muscles. This results in a significant increase in the intrathoracic pressure, which helps to prevent further air from entering the lungs during inspiration. Additionally, the closure of the glottis prevents air from escaping from the lungs during expiration. The residual volume (RV) is the volume of air remaining in the lungs after a normal expiration, and it is the point at which the lungs are at their most deflated state.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because this option does not accurately describe the end of normal expiration. The end of normal expiration is not characterized by the initiation of forced expiration.
**Option B:** Incorrect because the functional residual capacity (FRC) is the volume of air remaining in the lungs at the end of a passive expiration, not the end of a normal expiration.
**Option C:** Incorrect because the tidal volume (TV) is the volume of air inhaled or exhaled during normal breathing, not the volume of air remaining in the lungs at the end of a normal expiration.

**Clinical Pearl / High-Yield Fact**
The residual volume (RV) is an important concept in respiratory physiology, as it is the volume of air that cannot be measured directly during spirometry tests. Clinicians often estimate RV using indirect methods, such as the helium dilution technique or the body plethysmography method.

**Correct Answer: D**