**Core Concept**
Quiet exhalation, also known as exhalation at functional residual capacity, is a vital respiratory process where the diaphragm and accessory muscles work together to maintain lung volumes during relaxation. This process is essential for maintaining gas exchange and overall respiratory health.

**Why the Correct Answer is Right**
During quiet exhalation, the diaphragm relaxes and descends, reducing the volume of the thoracic cavity. This decrease in volume increases the intrathoracic pressure, facilitating the passive exhalation of air from the lungs. The accessory muscles, including the abdominis and internal intercostal muscles, also play a crucial role in this process by further decreasing the thoracic volume. The intercostal muscles specifically pull the ribs downward and inward, contributing to the reduction in thoracic volume.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the sternocleidomastoid muscle is primarily involved in forced inhalation and lateral flexion of the neck, not quiet exhalation.
**Option B:** Incorrect because the pectoralis major muscle is involved in adduction and medial rotation of the arm, not respiratory processes like quiet exhalation.
**Option C:** Incorrect because the latissimus dorsi muscle is involved in extension, adduction, and medial rotation of the arm, not quiet exhalation.

**Clinical Pearl / High-Yield Fact**
During quiet exhalation, the diaphragm's relaxation and descent are critical in maintaining lung volumes, and any impairment in this process can lead to respiratory failure. Understanding the role of accessory muscles in quiet exhalation is essential for diagnosing and managing respiratory disorders.

**Correct Answer:** D. Internal intercostal muscles