**Core Concept**
The spinal cord is a part of the central nervous system that extends from the base of the brain down to the lower back, but its length is limited by the closure of the neural tube during embryonic development. This closure results in the spinal cord ending at a specific level, which varies between individuals.

**Why the Correct Answer is Right**
The spinal cord ends at the level of L1-L2 in adults, which is below the level of the lumbar vertebrae. This is due to the differential growth rates of the spinal cord and the vertebral column during development. As the vertebral column grows, the spinal cord remains relatively short, resulting in its termination at the lower lumbar region. This is an important concept in understanding the anatomy of the spinal cord and its clinical implications.

**Why Each Wrong Option is Incorrect**
**Option A:** The spinal cord does not end at the T12 vertebra. The T12 vertebra is a part of the lumbar spine, but the spinal cord typically terminates at a lower level.
**Option B:** The spinal cord does not end at the S1 vertebra. The S1 vertebra is a part of the sacrum, and the spinal cord is not present in this region.
**Option C:** The spinal cord does not end at the C7 vertebra. The C7 vertebra is the lowest cervical vertebra, but the spinal cord extends below this level.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the spinal cord ends at a lower level than the vertebral column, which has implications for surgical and medical procedures involving the lower back.

**Correct Answer:** D. L1-L2.