**Core Concept**
The primary action of the inferior oblique muscle is to rotate the eyeball, specifically in the direction of abduction, elevation, and extorsion. This muscle plays a crucial role in the movement of the eye, especially in the upward and outward direction.

**Why the Correct Answer is Right**
The inferior oblique muscle primarily functions by rotating the eyeball in the direction of abduction, elevation, and extorsion. This action is achieved through its insertion on the sclera of the eyeball, which is positioned in such a way that it allows for the upward and outward movement of the eye. The muscle's primary action is also influenced by its innervation by the third cranial nerve, which provides the necessary motor control for its contraction.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as the primary action of the inferior oblique muscle is not adduction. Adduction is the movement of the eyeball towards the midline of the body, which is primarily achieved by the medial rectus muscle.

**Option B:** This option is incorrect as the primary action of the inferior oblique muscle is not depression. Depression is the movement of the eyeball downward, which is primarily achieved by the inferior rectus muscle.

**Option C:** This option is incorrect as the primary action of the inferior oblique muscle is not intorsion. Intorsion is the rotation of the eyeball inwards, which is primarily achieved by the superior oblique muscle.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the primary action of the inferior oblique muscle is to rotate the eyeball in the direction of abduction, elevation, and extorsion. This is crucial in understanding the movement of the eye and in diagnosing conditions that affect the extraocular muscles.

**Correct Answer: C. Extorsion.**