**Core Concept**
The rhomboid major muscle is a deep muscle of the back that plays a crucial role in scapular rotation and stabilization. The nerve supply to this muscle is essential for its proper function and innervation.

**Why the Correct Answer is Right**
The rhomboid major muscle receives its nerve supply from the dorsal scapular nerve, which is a branch of the brachial plexus (specifically, the C5 root). The dorsal scapular nerve arises from the ventral rami of the C5 root and enters the scapular region, where it innervates the rhomboid major and minor muscles. This nerve supply allows for coordinated movement and stabilization of the scapula during various activities such as lifting, pushing, and pulling.

**Why Each Wrong Option is Incorrect**
**Option A:** The nerve supply to the rhomboid major muscle is not provided by the long thoracic nerve, which innervates the serratus anterior muscle. The long thoracic nerve is responsible for the motor function of the serratus anterior, not the rhomboids.

**Option B:** The nerve supply to the rhomboid major muscle is not provided by the suprascapular nerve, which primarily innervates the supraspinatus and infraspinatus muscles. The suprascapular nerve does not have a significant role in the innervation of the rhomboid major muscle.

**Option C:** The nerve supply to the rhomboid major muscle is not provided by the thoracodorsal nerve, which innervates the latissimus dorsi muscle. The thoracodorsal nerve is responsible for the motor function of the latissimus dorsi, not the rhomboids.

**Clinical Pearl / High-Yield Fact**
Remember that the nerve supply to the rhomboid major muscle is distinct from the nerve supply to the serratus anterior muscle. This is essential for diagnosing and managing conditions such as scapular winging, which can be caused by damage to either the long thoracic nerve (serratus anterior) or the dorsal scapular nerve (rhomboid major).

**Correct Answer:** D.