**Core Concept**
The subcostal nerve is a branch of the thoracic nerves that arises from the 12th thoracic nerve (T12). It is a crucial nerve in the abdominal region, providing innervation to the abdominal muscles and skin.

**Why the Correct Answer is Right**
The subcostal nerve is the only nerve that emerges below the 12th rib, hence its name. It lies at the level of the lower border of the 12th rib, which is the lowest point of the rib cage. This nerve is a continuation of the thoracic plexus and plays a significant role in the innervation of the abdominal muscles, particularly the external oblique, internal oblique, and transversus abdominis muscles.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as the subcostal nerve does not lie at the level of the xiphoid process, which is located at the level of the 5th thoracic vertebra (T5).

**Option B:** This option is incorrect as the subcostal nerve does not lie at the level of the 10th thoracic vertebra (T10), which is too high for the subcostal nerve.

**Option C:** This option is incorrect as the subcostal nerve does not lie at the level of the 15th thoracic vertebra (T15), which is too low and does not exist.

**Clinical Pearl / High-Yield Fact**
The subcostal nerve is an important landmark in abdominal surgery, particularly in procedures such as hernia repair and laparoscopic surgery. Knowledge of the subcostal nerve's location is crucial to avoid nerve damage during surgical procedures.

**Correct Answer: C. at the level of the lower border of the 12th rib**