**Core Concept**
The protrusion of the tongue is a complex movement involving the coordinated action of several muscles in the tongue and surrounding structures. This movement is essential for various functions, including speech, swallowing, and manipulating food in the mouth.

**Why the Correct Answer is Right**
The correct answer involves the action of the extrinsic muscles of the tongue, specifically the **hypoglossus muscle**. When the hypoglossus muscle contracts, it protrudes the tongue by rotating it forward and outward. This muscle is innervated by the **hypoglossal nerve (CN XII)**, which is responsible for controlling the motor functions of the tongue. The contraction of the hypoglossus muscle also involves the action of other extrinsic muscles, such as the styloglossus and genioglossus muscles, which assist in the movement of the tongue.

**Why Each Wrong Option is Incorrect**
**Option A:** The styloglossus muscle is involved in retracting the tongue, not protruding it.
**Option B:** The genioglossus muscle assists in protruding the tongue, but it is not the primary muscle responsible for this action.
**Option C:** The palatoglossus muscle elevates the tongue and is involved in the process of swallowing, but it is not responsible for protruding the tongue.

**Clinical Pearl / High-Yield Fact**
The hypoglossal nerve (CN XII) is the only cranial nerve that is purely motor in function, meaning it only controls the movement of muscles and does not have any sensory or parasympathetic functions.

**Correct Answer: D. Hypoglossus muscle.**