Final answer:
The spinal cord is thicker in certain regions due to the density of nerves that control the limbs, and the highlighted thicker region is indicative of the area from which nerves supplying the lower limbs arise.
Step-by-step explanation:
The reason that the highlighted region of the spinal cord is thicker than surrounding areas is because nerves supplying the lower limbs arise from this region. The thickness in certain regions of the spinal cord is due to the number of nerve roots that come out from those areas to innervate specific parts of the body. In this context, the cervical and lumbar regions of the spinal cord show an increase in gray matter volume because they contain the cell bodies of motor neurons that supply the muscles of the arms and legs, respectively.
The spinal cord has enlargements in these regions that handle the neural circuitry for the limbs. These enlargements are structurally designed to handle the increased neural load necessary for limb movement and sensory processing. Therefore, option b is correct, indicating that this increase in thickness is associated with the greater number of nerves needed to serve the lower limbs.