Final answer:
The correct answer is b) At the level of S1; S4. In an adult, the spinal cord ends at the level of L2 and the dural sac extends to around S2.
Step-by-step explanation:
The adult spinal cord ends at the level of the first sacral vertebra (S1) and the dural sac extends even further to the level of the fourth sacral vertebra (S4).
The spinal cord is divided into regions that correspond to the regions of the vertebral column. The sacral region is located at the base of the vertebral column and includes the spinal cord at the level of the upper lumbar vertebral bones. So, the adult spinal cord ends at the level of L2 and the dural sac extends further to the level of S4.