**Core Concept**
The ulnar nerve is a branch of the brachial plexus that runs through the arm and forearm, supplying muscles and sensation to the hand. The ulnar nerve is responsible for controlling the intrinsic muscles of the hand, particularly those involved in fine motor movements.

**Why the Correct Answer is Right**
The ulnar nerve innervates the interosseous muscles, which are essential for finger spreading and adduction. It also provides sensation to the little finger and half of the ring finger. The ulnar nerve runs through the cubital tunnel in the elbow and the Guyon's canal in the wrist. Damage to the ulnar nerve can result in weakness or paralysis of the intrinsic muscles of the hand, leading to characteristic claw hand deformity.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the median nerve is responsible for innervating the thenar muscles, not the ulnar nerve.
**Option B:** This option is incorrect because the radial nerve is responsible for wrist extension and is not directly related to the ulnar nerve's function.
**Option C:** This option is incorrect because the musculocutaneous nerve is responsible for elbow flexion and is not related to the ulnar nerve's function.

**Clinical Pearl / High-Yield Fact**
The ulnar nerve is often compressed at the elbow, leading to numbness and tingling in the hand, particularly in the little finger and half of the ring finger. This condition is known as cubital tunnel syndrome.

**Correct Answer:** D.