**Core Concept**
The question is testing the student's knowledge of the anatomy and physiology of respiration, specifically the identification of the primary muscle responsible for inspiratory movements.

**Why the Correct Answer is Right**
The main inspiratory muscle is the diaphragm, which is a dome-shaped muscle located below the lungs. During inspiration, the diaphragm contracts and flattens, increasing the volume of the thoracic cavity and allowing air to enter the lungs. This movement is facilitated by the diaphragm's attachment to the xiphoid process and the lumbar vertebrae, allowing it to move downward and increase the negative pressure within the thoracic cavity.

**Why Each Wrong Option is Incorrect**
* **Option A:** The external intercostal muscles are involved in inspiration, but they are not the primary inspiratory muscle. They assist in the elevation of the ribs during inspiration.
* **Option B:** The internal intercostal muscles are involved in expiration, not inspiration. They help to depress the ribs and decrease the volume of the thoracic cavity.
* **Option C:** The accessory muscles of respiration, such as the sternocleidomastoid and scalene muscles, are involved in forced inspiration, but they are not the primary inspiratory muscle.

**Clinical Pearl / High-Yield Fact**
The diaphragm is the primary muscle of inspiration, accounting for up to 70% of the total inspiratory effort. Its innervation is provided by the phrenic nerve, which arises from the cervical spine.

**Correct Answer:** D. Diaphragm.