**Core Concept**
The question is testing the student's understanding of the anatomical and physiological basis of breathing. Specifically, it is asking about the muscle primarily responsible for expiratory movements. In humans, the process of breathing involves both inspiration and expiration, which are controlled by different muscles.

**Why the Correct Answer is Right**
The primary muscle responsible for expiration is the **diaphragm**. During expiration, the diaphragm contracts, increasing the intra-abdominal pressure, which in turn increases the pressure in the thoracic cavity. This increased pressure pushes the air out of the lungs, allowing for expiration. The diaphragm plays a crucial role in regulating breathing, particularly during quiet, relaxed breathing.

**Why Each Wrong Option is Incorrect**
* **Option A:** The **sternocleidomastoid** is a muscle of the neck involved in rotational movements of the head and neck, not expiration.
* **Option B:** The **trapezius** is a muscle of the upper back involved in shoulder movements, not expiration.
* **Option C:** The **psoas major** is a muscle of the lower back involved in hip flexion, not expiration.

**Clinical Pearl / High-Yield Fact**
The diaphragm is the primary muscle of inspiration, and it is responsible for approximately 75% of the volume of air inhaled during quiet breathing. The diaphragm is a dome-shaped muscle that separates the chest cavity from the abdominal cavity.

**Correct Answer: D. Diaphragm.**