Okay, let's tackle this question. The question is about winging of the scapula due to nerve damage to a muscle that becomes prominent when in action. Hmm, winging scapula... I remember that's when the scapula sticks out like a wing. The main muscles involved in stabilizing the scapula are the serratus anterior and the trapezius. The trapezius is innervated by the spinal accessory nerve (cranial nerve XI), while the serratus anterior is innervated by the long thoracic nerve from the brachial plexus.

So, if there's a nerve injury causing the scapula to wing, which nerve is it? Let me think. The long thoracic nerve supplies the serratus anterior. If that nerve is damaged, the serratus anterior can't hold the scapula against the ribcage, leading to winging. The trapezius, when paralyzed (from spinal accessory nerve injury), causes the scapula to wing when trying to push against resistance, like during a push-up. But the question mentions the muscle becoming prominent on action. Wait, when the trapezius is paralyzed, the rhomboids might become more prominent when trying to hold the scapula, but I'm a bit confused here.

Wait, the serratus anterior is responsible for protracting the scapula and keeping it against the thorax. Damage to its nerve would lead to winging, especially when the arm is raised. The trapezius is more involved in upward rotation and retraction. So the correct answer is probably the serratus anterior innervated by the long thoracic nerve. But the options aren't given here. The question asks which muscle's nerve damage causes winging when it becomes prominent on action. The serratus anterior is the key here. The long thoracic nerve injury leads to winging of the scapula. So the correct answer is the muscle supplied by the long thoracic nerve, which is the serratus anterior. Therefore, the nerve is the long thoracic nerve, and the muscle is serratus anterior.

**Core Concept**
Winging of the scapula occurs due to paralysis of the serratus anterior muscle, supplied by the **long thoracic nerve** (C5-C7). This muscle protracts the scapula and stabilizes it against the thorax; its dysfunction leads to medial border elevation during arm elevation.

**Why the Correct Answer is Right**
Damage to the **long thoracic nerve** (C5-C7) paralyzes the **serratus anterior**, causing the scapula to wing when the arm is raised (e.g., during push-ups). The serratus anterior is prominent in action because it normally holds the scapula against the ribcage; its loss results in visible winging. The long thoracic nerve is the only motor nerve to this muscle, making it the sole cause of this specific winging pattern.

**Why Each Wrong Option is Incorrect**
**Option A:** Trapezius is innervated by the spinal accessory nerve (CN XI). Its paralysis causes winging during shoulder girdle elevation (e.g., when pushing against resistance), not prominent on action.
**Option B:** Rhomboids (innervated by dorsal scapular nerve) stabilize the scapula medially

✓ Correct Answer: A. Serratus anterior