**Core Concept**
The dual nerve supply of muscles is a phenomenon where a muscle receives innervation from two separate nerve roots or branches, which can provide redundancy and compensation in case of nerve injury. This is particularly important in the context of anatomy and physiology, where understanding the innervation of muscles is crucial for diagnosing and treating various neurological conditions.

**Why the Correct Answer is Right**
The muscles of the pelvic floor, including the levator ani and the coccygeus, receive dual nerve supply from the sacral spinal nerves (S3-S4) and the pudendal nerve, which arises from S2-S4. This dual innervation is essential for maintaining continence and supporting pelvic organ function. The pudendal nerve provides motor innervation to the levator ani and coccygeus muscles, while the sacral spinal nerves provide sensory innervation. This complex innervation pattern allows for coordinated control of pelvic floor muscles.

**Why Each Wrong Option is Incorrect**
**Option A:** The rectus abdominis muscle receives innervation from the thoracoabdominal nerves (T6-T12), which provide motor innervation to this muscle. This is not a dual nerve supply, as it is a single nerve root that provides innervation.
**Option B:** The external anal sphincter muscle receives innervation from the pudendal nerve, which provides motor innervation to this muscle. This is not a dual nerve supply, as it is a single nerve root that provides innervation.
**Option C:** The diaphragm muscle receives innervation from the phrenic nerve (C3-C5) and the lower intercostal nerves (T6-T12), which provide motor innervation to this muscle. This is a dual nerve supply, making it a correct example of dual innervation.

**Clinical Pearl / High-Yield Fact**
The pelvic floor muscles, including the levator ani and coccygeus, have a unique dual nerve supply that provides redundancy and compensation in case of nerve injury. Understanding this complex innervation pattern is essential for diagnosing and treating various neurological conditions, such as fecal incontinence and pelvic organ prolapse.

**Correct Answer:** C. The diaphragm muscle does not have dual nerve supply except the phrenic nerve and lower intercostal nerves.