Final answer:
The spinal cord is the lowest level of the CNS.
Step-by-step explanation:
The correct answer is d. spinal cord. The spinal cord is the lowest level of the Central Nervous System (CNS). It is a long, thin bundle of nerve fibers that extends from the base of the brain to the lower back. It is responsible for transmitting sensory information from the body to the brain and motor signals from the brain to muscles and organs.