**Core Concept**
The question requires knowledge of the motor and sensory functions of the median and ulnar nerves in the hand. The median nerve controls the muscles of the thumb, including opposition, while the ulnar nerve controls the muscles of the little finger. The inability of the thumb to touch the tip of the little finger suggests an impairment of opposition.

**Why the Correct Answer is Right**
Opposition is the movement of the thumb against the other fingers, allowing for grasping and fine motor functions. The median nerve supplies the thenar muscles, which are responsible for opposition. Damage to this nerve would result in weakness or paralysis of the thenar muscles, leading to the inability of the thumb to touch the tip of the little finger. The palmar cutaneous branch of the median nerve may also be involved.

**Why Each Wrong Option is Incorrect**
**Option A:** The radial nerve primarily controls extension of the wrist and fingers, not opposition or flexion of the thumb.
**Option B:** The ulnar nerve controls adduction of the little finger, not opposition of the thumb.
**Option C:** The musculocutaneous nerve primarily controls flexion of the elbow and forearm muscles, not fine motor functions of the hand.

**Clinical Pearl / High-Yield Fact**
The "anatomical snuff box" is a significant landmark on the radial side of the wrist, bordered by the tendons of the extensor pollicis longus, extensor pollicis brevis, and abductor pollicis longus. Damage to the radial nerve can cause weakness in this area, making it an important clinical examination point.

**Correct Answer: B. Ulnar nerve.**