**Core Concept**
The dorsal interosseous muscles are a group of muscles in the hand that play a crucial role in finger movements, particularly abduction and extension. These muscles are innervated by the ulnar nerve, which is responsible for their motor function.

**Why the Correct Answer is Right**
The fourth dorsal interosseous muscle receives its motor innervation from the ulnar nerve. This nerve supplies the majority of the intrinsic muscles of the hand, including the interosseous muscles, which enable precise movements of the fingers. The ulnar nerve's motor branches, specifically the deep branches, innervate these muscles, allowing for the coordinated movement of the fingers.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the median nerve, not the ulnar nerve, primarily innervates the thenar muscles of the hand.
* **Option B:** This option is incorrect because the radial nerve, which supplies the extensor muscles of the wrist and fingers, does not innervate the dorsal interosseous muscles.

**Clinical Pearl / High-Yield Fact**
The ulnar nerve's involvement in the innervation of the interosseous muscles is crucial for understanding hand function and diagnosing ulnar nerve entrapment or compression syndromes, such as cubital tunnel syndrome.

**Correct Answer:** C. The ulnar nerve.