**Core Concept**
The axillary nerve is a branch of the brachial plexus, a complex network of nerves that originates from the spinal cord and controls various functions of the upper limb. The brachial plexus is formed by the ventral rami of the C5 to T1 spinal nerves.

**Why the Correct Answer is Right**
The axillary nerve arises from the posterior cord of the brachial plexus, which is formed by the union of the C5 and C6 spinal nerves. Specifically, it is a branch of the posterior cord that runs through the quadrilateral space and innervates the deltoid and teres minor muscles. The axillary nerve is responsible for motor innervation of the deltoid and teres minor muscles, as well as sensory innervation of the skin over the deltoid region.

**Why Each Wrong Option is Incorrect**
**Option A:** The musculocutaneous nerve is a branch of the lateral cord of the brachial plexus, not the posterior cord. It innervates the biceps brachii, brachialis, and coracobrachialis muscles.
**Option B:** The radial nerve is a branch of the posterior cord of the brachial plexus, but it is not the correct answer. The radial nerve innervates the triceps brachii and supplies the skin over the back of the arm.
**Option C:** The ulnar nerve is a branch of the medial cord of the brachial plexus, not the posterior cord. It innervates the flexor carpi ulnaris, flexor digitorum profundus, and intrinsic muscles of the hand.

**Clinical Pearl / High-Yield Fact**
The axillary nerve is at risk of injury during procedures such as shoulder surgery or axillary dissection, which can result in weakness or paralysis of the deltoid and teres minor muscles.

**Correct Answer: C. Posterior cord.**