**Core Concept**
The cranial vault is a critical structure that houses the brain, and its volume is essential for proper brain development and function. The cranial vault volume increases significantly during childhood, particularly during the first 7 years, as the brain grows and matures.

**Why the Correct Answer is Right**
The cranial vault volume at 7 years is approximately 90% of its adult size. This rapid growth is due to the proliferation and differentiation of brain cells, including neurons and glial cells. The increase in cranial vault volume allows for the expansion of brain tissue and the development of complex neural connections.

**Why Each Wrong Option is Incorrect**
* **Option A:** Incorrect because cranial vault volume at birth is only about 60% of its adult size, not at 7 years.
* **Option B:** Incorrect because cranial vault volume at 1 year is about 70% of its adult size, not at 7 years.
* **Option C:** Incorrect because cranial vault volume at 7 years is not 80% of its adult size, but rather 90%.

**Clinical Pearl / High-Yield Fact**
The cranial vault volume increases by about 1-2% per year during the first 7 years of life, making it a critical period for brain development and growth.

**Correct Answer:** C.