**Core Concept**
The spinal cord, a continuation of the brainstem, extends from the base of the brain to the lower border of the first lumbar vertebra (L1) in adults. This level is known as the conus medullaris, which marks the end of the spinal cord's anatomical structure. Below this point, the spinal canal contains only the cauda equina, a collection of nerve roots.

**Why the Correct Answer is Right**
The spinal cord terminates at the lower border of L1 due to the differential growth rates of the vertebral column and the spinal cord. As the vertebral column grows, the spinal cord remains relatively short, resulting in the conus medullaris being located at L1. This level is consistent across most adults and serves as a crucial anatomical landmark in medical procedures and imaging studies.

**Why Each Wrong Option is Incorrect**
* **Option B:** The tip of the coccyx is actually the location of the cauda equina, which consists of nerve roots that arise from the lower lumbar and sacral spinal cord segments. This is below the level of the conus medullaris.
* **Option C:** The upper border of L3 is too high to mark the end of the spinal cord, as the conus medullaris is typically located at L1 in adults.
* **Option D:** S2 is a sacral vertebra, and the spinal cord does not extend below the level of L1.

**Clinical Pearl / High-Yield Fact**
The conus medullaris is a critical anatomical landmark in medical procedures, such as spinal anesthesia and lumbar punctures. It is essential to identify the correct level to avoid injury to the spinal cord.

**✓ Correct Answer: A. Lower border of L1**