**Core Concept:** Voluntary extraocular muscles are those that control the movement of the eye in various directions, while the extraocular muscles are involuntary and responsible for maintaining the position of the eye.

**Why the Correct Answer is Right:** The correct answer, **D**, refers to the **inferior oblique muscle (IO)**, which is a voluntary muscle responsible for torsional movements of the eye, particularly downward and medial rotation. In contrast, the involuntary extraocular muscles are responsible for maintaining the eye's position and are not the focus of this question.

**Why Each Wrong Option is Incorrect:**

A. **Rectus muscles (RA, RL, RE, RM, RD, RH):** These are the six voluntary extraocular muscles responsible for the primary eye movements, including up, down, left, right, in, and out (upon convergence). They are not involuntary.

B. **Oculomotor nerve (CN III):** The oculomotor nerve controls the three extraocular muscles (rectus, oblique, and lateral rectus) responsible for voluntary eye movements. It is incorrect to label the entire nerve as involuntary.

C. **Avoid mentioning option C here, as it is the correct answer.**

D. **Inferior oblique muscle (IO):** This is a voluntary muscle responsible for torsional movements of the eye, particularly downward and medial rotation.

5. **Clinical Pearl:** Understanding the distinction between voluntary and involuntary extraocular muscles is crucial for understanding eye movements and the anatomy of the extraocular muscles. This knowledge is essential for diagnosing and treating ocular motility disorders, such as strabismus and diplopia.

**Correct Answer:** D (inferior oblique muscle)