**Core Concept**
The question is testing the knowledge of the motor innervation of the hand, specifically the function of opposing the thumb and little finger. The radial and ulnar nerves are responsible for the motor innervation of the hand, with the radial nerve controlling the extensor muscles and the ulnar nerve controlling the flexor muscles.

**Why the Correct Answer is Right**
The ulnar nerve is responsible for the motor innervation of the intrinsic muscles of the hand, including the opponents pollicis, which opposes the thumb, and the opponents digiti minimi, which opposes the little finger. The ulnar nerve injury would lead to weakness or paralysis of these muscles, resulting in the inability to oppose the tips of the thumb and little finger. The ulnar nerve runs through the Guyon's canal at the wrist, making it susceptible to injury from a wound in this area.

**Why Each Wrong Option is Incorrect**
**Option A:** The median nerve is responsible for the motor innervation of the thenar muscles, including the abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis, but not the opponents digiti minimi.
**Option B:** The radial nerve is responsible for the motor innervation of the extensor muscles of the wrist and fingers, but not the intrinsic muscles of the hand.
**Option C:** The musculocutaneous nerve is responsible for the motor innervation of the biceps brachii, brachialis, and coracobrachialis muscles, but not the muscles of the hand.

**Clinical Pearl / High-Yield Fact**
The ulnar nerve is often injured in wrist injuries due to its proximity to the wrist joint and the Guyon's canal. A thorough examination of the hand and wrist is essential in diagnosing ulnar nerve injuries.

**Correct Answer: C. Ulnar nerve.**