## Core Concept
The question tests knowledge of musculoskeletal anatomy, specifically focusing on the nerve supply to various muscles. A dual nerve supply implies that a muscle receives innervation from more than one nerve.

## Why the Correct Answer is Right
The muscle with a dual nerve supply is the **Trapezius**. It primarily receives its nerve supply from the **cranial nerve XI (Accessory nerve)**, but it also receives proprioceptive fibers from the **C3 and C4 spinal nerves**. This dual innervation allows for a wide range of motion and functional versatility.

## Why Each Wrong Option is Incorrect
* **Option A:** The Sternocleidomastoid muscle primarily receives its nerve supply from the **Accessory nerve (CN XI)**, similar to the Trapezius, but it does not have a well-documented dual nerve supply like the Trapezius.
* **Option B:** The Pectoralis major muscle receives its nerve supply from the **Lateral and medial pectoral nerves**, which are branches of the brachial plexus. While it does have a dual nerve supply, the context of the question likely seeks a muscle with a more distinctly dual nerve supply involving a cranial nerve and spinal nerves.
* **Option D:** The Deltoid muscle primarily receives its nerve supply from the **Axillary nerve**, a branch of the brachial plexus. It does not have a documented dual nerve supply.

## Clinical Pearl / High-Yield Fact
A key clinical point to remember is that the **Trapezius muscle's dual innervation** from both the accessory nerve (CN XI) and the cervical spinal nerves (C3-C4) is crucial for its function in movements of the scapula and shoulder. Damage to either of these nerve supplies can lead to weakness and altered movement patterns.

## Correct Answer: C. Trapezius