**Core Concept**
The patient's inability to extend his forearm following a humerus fracture is due to damage to the radial nerve, which is responsible for controlling the extensor muscles of the forearm. The radial nerve originates from the posterior cord of the brachial plexus and is composed of fibers from multiple spinal levels.

**Why the Correct Answer is Right**
The radial nerve is formed from the combination of nerve fibers from the C5 to C8 spinal levels, which are part of the posterior cord of the brachial plexus. These fibers then merge to form the radial nerve, which descends through the arm and provides motor innervation to the muscles of the posterior forearm, including the extensor carpi radialis brevis and extensor carpi radialis longus. Damage to the radial nerve, such as that caused by a humerus fracture, can result in weakness or paralysis of these muscles, leading to the inability to extend the forearm.

**Why Each Wrong Option is Incorrect**
* **Option A:** The median nerve is primarily responsible for controlling the muscles of the anterior forearm, including flexor muscles, and does not contribute to the radial nerve.
* **Option B:** The ulnar nerve is responsible for controlling the muscles of the medial forearm and hand, including the flexor carpi ulnaris and the interosseous muscles, and does not contribute to the radial nerve.
* **Option C:** The musculocutaneous nerve primarily innervates the muscles of the anterior arm, including the biceps brachii and brachialis, and does not contribute to the radial nerve.

**Clinical Pearl / High-Yield Fact**
The radial nerve is often damaged in fractures of the mid-to-distal humerus, particularly the radial head or neck, which can result in a characteristic "wrist drop" deformity due to the inability to extend the wrist or fingers.

**Correct Answer: C. C5-C8.**