**Core Concept**
The sulci are the grooves on the surface of the brain that separate the gyri. A complete sulcus is one that extends from the medial surface of the brain to the lateral surface.

**Why the Correct Answer is Right**
A complete sulcus is formed by the infolding of the brain's surface, creating a groove that separates the gyri. The sulci are formed during fetal development and continue to deepen as the brain matures. The complete sulci, such as the central sulcus, are essential for the separation of the frontal, parietal, and temporal lobes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not specified, so we cannot evaluate its correctness.
**Option B:** This option is also not specified, so we cannot evaluate its correctness.
**Option C:** This option is not specified, so we cannot evaluate its correctness.
**Option D:** This option is also not specified, so we cannot evaluate its correctness.

**Clinical Pearl / High-Yield Fact**
The sulci and gyri are dynamic structures that can change in response to various neurological conditions, such as Alzheimer's disease, which is characterized by the loss of neurons and the widening of the sulci.

**Correct Answer:** C.