**Core Concept:** Functional residual capacity (FRC) is a measure of the amount of air remaining in the lungs at rest, after expiration has been completed but before a full inspiration. It is an essential component of total lung capacity (TLC) and helps maintain normal gas exchange.

**Why the Correct Answer is Right:** Functional residual capacity is the volume of air remaining in the lungs after a complete exhalation but before a full inhalation, which is crucial for maintaining normal gas exchange in the lungs. In this case, the correct answer is **C**:

**Correct Answer: Total lung capacity (TLC)**

**Why Each Wrong Option is Incorrect:**

A. **Lowest lung volume:** This option is incorrect because FRC is not the lowest lung volume but rather the volume after complete exhalation before full inhalation.
B. **Highest lung volume:** This option is wrong as FRC is not the highest lung volume, but rather the volume after exhalation before full inhalation.
D. **Partial inspiratory capacity (PIC):** Partial inspiratory capacity is the volume of air inhaled after a partial exhalation, whereas FRC is the volume after complete exhalation before full inhalation.

**Clinical Pearl:** Understanding FRC and its relationship to TLC is essential for medical professionals, as it helps in identifying lung volumes that may indicate respiratory diseases like pulmonary fibrosis, which can lead to reduced FRC, or pulmonary edema, causing increased FRC.