**Core Concept**
The stapedius muscle is a small skeletal muscle in the middle ear, responsible for stabilizing the stapes bone and regulating sound intensity. Its innervation is crucial for maintaining balance and hearing.

**Why the Correct Answer is Right**
The correct answer is the 7th nerve, also known as the facial nerve (cranial nerve VII). The facial nerve has a complex branch called the nerve to the stapedius muscle, which provides motor innervation to the stapedius muscle. This innervation allows the stapedius muscle to contract and relax in response to sound stimuli, thereby stabilizing the stapes bone and regulating sound intensity. The facial nerve also carries taste fibers from the anterior two-thirds of the tongue and provides motor innervation to the muscles of facial expression.

**Why Each Wrong Option is Incorrect**
**Option A:** The 5th nerve, or trigeminal nerve (cranial nerve V), is responsible for sensory innervation of the face and motor innervation of the muscles of mastication, but it does not supply the stapedius muscle.
**Option B:** The 6th nerve, or abducens nerve (cranial nerve VI), is responsible for motor innervation of the lateral rectus muscle, which rotates the eye outward, but it does not supply the stapedius muscle.
**Option D:** The 8th nerve, or vestibulocochlear nerve (cranial nerve VIII), is responsible for transmitting sound and balance information from the inner ear to the brain, but it does not provide motor innervation to the stapedius muscle.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the stapedius muscle is the only skeletal muscle in the middle ear, and its innervation by the facial nerve is crucial for maintaining normal hearing and balance.

**✓ Correct Answer: C. 7th nerve. The stapedius muscle is supplied by the facial nerve (cranial nerve VII).**