Okay, let's tackle this question. The patient has acinic cell carcinoma in the submandibular region. After surgery, his tongue deviates to the affected side. The question is about which nerve was damaged.

First, I need to recall the anatomy of the tongue and the muscles involved. The tongue's movement is controlled by several muscles. The primary extrinsic muscles are the genioglossus and styloglossus. The genioglossus is responsible for protruding and deviating the tongue. If the genioglossus is damaged, the tongue would deviate towards the affected side because the contralateral genioglossus would dominate.

Now, the genioglossus is innervated by the hypoglossal nerve (CN XII). So if the hypoglossal nerve is damaged during surgery, the genioglossus on that side wouldn't function, leading to deviation towards the affected side. That makes sense because the healthy side's genioglossus would pull the tongue away from the damaged side.

Let me check the other options. The facial nerve (CN VII) controls the facial muscles and some minor tongue muscles like the stylohyoid, but not the genioglossus. The glossopharyngeal nerve (CN IX) is more involved in sensation and taste, not motor. The vagus nerve (CN X) has some motor functions in the pharynx but not the tongue.

So the correct answer should be the hypoglossal nerve (CN XII). The key here is remembering that tongue deviation due to paralysis is a classic sign of hypoglossal nerve injury. The other nerves don't innervate the genioglossus, so their damage wouldn't cause this specific symptom.

**Core Concept**
This question tests knowledge of cranial nerve anatomy and its clinical implications, specifically the hypoglossal nerve (CN XII) and its role in tongue movement. The genioglossus muscle, innervated by CN XII, is responsible for protruding the tongue; unilateral injury causes deviation toward the affected side due to unopposed contralateral muscle action.

**Why the Correct Answer is Right**
The hypoglossal nerve (CN XII) directly innervates the genioglossus muscle. Damage to CN XII during submandibular surgery (e.g., near the hypoglossal nerve’s course through the neck) results in paralysis of the genioglossus. When the patient protrudes the tongue, the intact contralateral genioglossus dominates, pulling the tongue toward the affected side. This is a hallmark of hypoglossal nerve injury.

**Why Each Wrong Option is Incorrect**
**Option A:** The facial nerve (CN VII) innervates facial muscles but not the genioglossus. Damage here would cause facial weakness, not tongue deviation.
**Option B:** The glossopharyngeal nerve (CN IX) provides sensory input to the tongue’s posterior third and contributes to swallowing but does not control tongue protrusion.
**Option C:** The vagus nerve (CN X) aids in pharyngeal muscles and swallowing but not direct tongue deviation.

**Clinical Pearl / High-Yield Fact**
Remember the **"tongue deviation rule":** unilateral hypoglossal nerve injury causes the tongue

✓ Correct Answer: C. Hypoglossal nerve