**Core Concept**
The posterior longitudinal ligament (PLL) is a crucial ligament in the cervical spine, playing a significant role in maintaining the structural integrity of the vertebral column. It is a fibroelastic ligament that extends from the base of the skull to the sacrum, providing support and stability to the spine.

**Why the Correct Answer is Right**
The PLL is a continuous ligament that runs along the posterior aspect of the vertebral bodies, from the base of the skull (C1) to the sacrum (S5). It is attached to the anterior surface of the vertebral arches and the posterior surface of the vertebral bodies, forming a strong bond between the vertebrae. The PLL is essential for maintaining the normal curvature of the spine and preventing excessive movement or instability.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as the PLL does not continue as the anterior longitudinal ligament, which is a separate ligament that runs along the anterior aspect of the vertebral bodies.

**Option B:** This option is incorrect as the PLL does not continue as the supraspinous ligament, which is a separate ligament that runs along the posterior aspect of the spine, but is located more superficially than the PLL.

**Option C:** This option is incorrect as the PLL does not continue as the interspinous ligament, which is a separate ligament that runs between the spinous processes of adjacent vertebrae.

**Clinical Pearl / High-Yield Fact**
The PLL is a strong ligament that can be a source of pain and discomfort if it is injured or irritated, such as in cases of spinal stenosis or spondylolisthesis.

**Correct Answer: C. Ligamentum flavum**