**Core Concept**
Prolapsed intervertebral disc (PID) occurs when the soft, gel-like center of an intervertebral disc bulges out through a tear in the outer, tougher layer. This condition often results from degenerative changes, trauma, or repetitive strain on the spine.

**Why the Correct Answer is Right**
The most common level for PID is L4-L5 due to the combination of biomechanical factors and anatomical characteristics. The L4-L5 disc is subjected to significant stress and strain, particularly in individuals with sedentary occupations or those who perform repetitive bending and twisting. The L4-L5 disc is also more prone to degeneration due to its relatively low mobility and high stress, making it a common site for disc prolapse.

**Why Each Wrong Option is Incorrect**
* **Option A:** While the cervical spine can be affected by PID, it is less common than lumbar disc prolapse. The cervical spine is more mobile and has a thicker annulus fibrosus, making it less susceptible to disc prolapse.
* **Option B:** This option is incorrect as the thoracic spine is less commonly affected by PID. The thoracic spine has a more rigid and less mobile segment, which reduces the likelihood of disc prolapse.
* **Option D:** This option is incorrect as the L5-S1 disc can be affected by PID, but it is less common than L4-L5 disc prolapse. The L5-S1 disc is subjected to significant stress, but its relatively lower mobility and thicker annulus fibrosus make it less prone to disc prolapse.

**Clinical Pearl / High-Yield Fact**
Remember the "Danger Zone" for disc prolapse: L4-L5 and L5-S1. These two levels are the most common sites for disc prolapse due to a combination of biomechanical factors, anatomical characteristics, and degenerative changes.

**Correct Answer:** C. L4-L5