**Core Concept:** Teres major is a muscle located in the posterior lateral aspect of the upper arm, and it is innervated by specific nerves. In the context of the multiple-choice question, we need to identify the nerve supply of Teres major.

**Why the Correct Answer is Right:** Teres major is innervated by the posterior cord of the brachial plexus. The posterior cord arises from the ventral rami of C5, C6, and C7 spinal nerves, and supplies the muscles in the posterior compartment of the upper limb, including the teres major muscle.

**Why Each Wrong Option is Incorrect:**

A. **Teres minor:** Both Teres major and Teres minor are supplied by the posterior cord of the brachial plexus, but they differ in their innervation. Teres minor is supplied by C8 and T1 spinal nerves, not C5, C6, and C7.

B. **Suprascapular nerve:** The suprascapular nerve arises from the posterior cord of the brachial plexus and supplies the supraspinatus muscle. Teres major is supplied by the posterior cord, not the suprascapular nerve.

C. **Axillary nerve:** The axillary nerve is another branch of the brachial plexus that supplies the arm muscles. However, it does not supply the teres major muscle; instead, it innervates the deltoid and biceps muscles.

D. **Long thoracic nerve:** The long thoracic nerve is a branch of the spinal accessory nerve (CN XI), which is different from the posterior cord of the brachial plexus. The long thoracic nerve supplies the serratus anterior muscle, not teres major.

**Clinical Pearl:** The understanding of the anatomy and innervation of upper limb muscles is crucial for safe and effective surgical procedures and clinical practice involving the shoulder and arm, as well as for recognizing potential complications such as nerve injuries.