Final answer:
The spinal cord terminates at the lower border of the L1 vertebra in most adults, which is the correct answer for where the spinal cord ends.
Step-by-step explanation:
In adults, the spinal cord typically terminates at the level of the upper lumbar vertebrae. More precisely, the spinal cord ends at the lower border of the L1 vertebra and in some individuals, it may extend slightly lower to the upper border of the L2 vertebra. After this point, the nerve roots that originate from the spinal cord extend downward to form the cauda equina, a bundle of nerves resembling a horse's tail, which occupies the lumbar and sacral regions of the vertebral column.
Hence, the correct answer to the question is A. L1.