**Core Concept**
The question tests the dermatomal distribution of the thoracic spinal nerves. Dermatomes are areas of skin that are supplied by nerves that arise from specific spinal nerve roots. Each thoracic spinal nerve has a corresponding dermatome that covers a specific area of the skin on the anterior trunk.

**Why the Correct Answer is Right**
The skin over the xiphoid process is innervated by the 7th thoracic spinal nerve (T7). This is because the dermatomal distribution of the thoracic spinal nerves follows a predictable pattern, with each nerve root giving rise to a dermatome that covers a specific area of the skin. The T7 dermatome corresponds to the area around the xiphoid process.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the correct thoracic spinal nerve.
**Option B:** This option is incorrect because it does not correspond to the dermatomal distribution of the thoracic spinal nerves.
**Option D:** This option is incorrect because it is not a thoracic spinal nerve.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the dermatomal distribution of the thoracic spinal nerves can be used to diagnose nerve root lesions. For example, a patient with pain or numbness in the T7 dermatome may have a lesion affecting the 7th thoracic spinal nerve.

**Correct Answer:** C. T7