**Core Concept**
The ulnar nerve innervates several important muscles and functions in the hand, specifically those involved in fine motor movements and sensation. A complete section of the ulnar nerve at the wrist would result in the paralysis of these muscles and functions.

**Why the Correct Answer is Right**
The ulnar nerve innervates the intrinsic muscles of the hand, including the interosseous muscles, the lumbricals to the ring and little fingers, and the adductor pollicis muscle. The paralysis of these muscles would lead to the characteristic "claw hand" deformity, where the ring and little fingers are drawn into the palm. This is due to the loss of opposition and fine motor movements in the hand. The ulnar nerve also provides sensation to the little finger and the ulnar half of the ring finger, leading to numbness and tingling in these areas.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the median nerve, not the ulnar nerve, innervates the thenar muscles, which include the abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis muscles.

**Option B:** This option is incorrect because the radial nerve, not the ulnar nerve, innervates the extensor muscles of the wrist and fingers, including the extensor carpi radialis brevis, extensor carpi radialis longus, and extensor digitorum muscles.

**Option C:** This option is incorrect because the musculocutaneous nerve, not the ulnar nerve, innervates the biceps brachii, brachialis, and coracobrachialis muscles, which are involved in elbow flexion and shoulder stabilization.

**Clinical Pearl / High-Yield Fact**
In a patient with a complete ulnar nerve section, the "claw hand" deformity can be diagnosed by asking the patient to place their hand flat on a surface and then lifting their fingers off the surface. If the ring and little fingers remain in the palm, it suggests ulnar nerve paralysis.

**Correct Answer: D. Intrinsic muscles of the hand.**