**Core Concept**
The superior oblique muscle is one of the six extraocular muscles responsible for controlling eye movements. It plays a crucial role in the rotation of the eyeball, particularly in the downward and inward movement.

**Why the Correct Answer is Right**
The superior oblique muscle is innervated by the trochlear nerve, which is the fourth cranial nerve (CN IV). This nerve arises from the dorsal aspect of the brainstem, specifically from the trochlear nucleus. The trochlear nerve then exits the brainstem and enters the orbit through the superior orbital fissure, where it innervates the superior oblique muscle. The motor fibers of the trochlear nerve control the contraction of the superior oblique muscle, enabling its functions in eye movement.

**Why Each Wrong Option is Incorrect**
**Option A:** The 3rd cranial nerve, also known as the oculomotor nerve, supplies four of the six extraocular muscles (superior, inferior, medial rectus, and inferior oblique) but not the superior oblique muscle.

**Option C:** The 5th cranial nerve, or the trigeminal nerve, is primarily responsible for sensory innervation of the face and motor innervation of the muscles of mastication. It does not innervate any of the extraocular muscles.

**Option D:** The 6th cranial nerve, or the abducens nerve, supplies the lateral rectus muscle, which is responsible for outward eye movement. It does not innervate the superior oblique muscle.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the trochlear nerve (4th cranial nerve) has the longest intracranial course among all the cranial nerves, making it more susceptible to injury. This knowledge can be critical in diagnosing and managing patients with trochlear nerve palsy.

**✓ Correct Answer: B. 4th cranial nerve**