**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 rotational movements of the eye, particularly in the depression and intorsion of the eyeball.

**Why the Correct Answer is Right**
The superior oblique muscle is innervated by the trochlear nerve (cranial nerve IV) and is primarily responsible for the intorsion (rotating the top of the eyeball towards the nose) and depression of the eyeball. This is achieved through its unique insertion on the sclera of the eye, which allows it to pull the eyeball in a specific direction. The muscle's action is most evident when the eye is adducted (moved towards the midline of the body) and the head is in a state of upward gaze.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the superior oblique muscle does not primarily function to abduct the eyeball. Abduction is the movement of the eye away from the midline of the body, which is primarily facilitated by the lateral rectus muscle.
* **Option B:** This option is incorrect because the superior oblique muscle does not primarily function to elevate the eyeball. Elevation is the movement of the eye upwards, which is primarily facilitated by the superior rectus muscle.
* **Option C:** This option is incorrect because the superior oblique muscle does not primarily function to extort the eyeball. Extorsion is the rotation of the top of the eyeball away from the nose, which is primarily facilitated by the inferior oblique muscle.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the superior oblique muscle is the only extraocular muscle that is innervated by a cranial nerve that decussates (crosses over) before exiting the brainstem. This unique characteristic can be useful in diagnosing trochlear nerve palsy.

**Correct Answer: C. Depression and intorsion of the eyeball.**