**Core Concept**
The long thoracic nerve is a motor nerve that innervates the serratus anterior muscle, playing a crucial role in shoulder movement and stabilization. The root value of a nerve refers to the specific spinal nerve root that contributes to its formation.

**Why the Correct Answer is Right**
The long thoracic nerve is formed by the spinal nerve roots C5, C6, and C7. These roots arise from the anterior rami of the corresponding cervical spinal nerves. The contributions of these roots to the long thoracic nerve are not equal, with C5 being the primary contributor. The nerve then courses downward to innervate the serratus anterior muscle, which is essential for rotating the scapula and elevating the arm.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect, as it does not accurately represent the primary root value of the long thoracic nerve.
**Option B:** Incorrect, as it excludes one of the primary contributors to the nerve.
**Option C:** Incorrect, as it includes an additional root that is not typically associated with the long thoracic nerve.

**Clinical Pearl / High-Yield Fact**
The long thoracic nerve is vulnerable to injury during surgical procedures, such as mastectomies or thoracotomies, which can result in winged scapula due to serratus anterior paralysis.

**Correct Answer: C. C5, C6, C7**