Option 1 states that only blood constitutes the central nervous system (CNS), which is incorrect. The CNS consists of the brain and spinal cord, not just blood. Option 2 states that only the brain constitutes the CNS, which is also incorrect. While the brain is a crucial part of the CNS, it is not the only component. Option 3 states that only the spinal cord constitutes the CNS, which is once again incorrect. The spinal cord is an essential part of the CNS, but it is not the sole component. Option 4 correctly states that both the brain and spinal cord together constitute the CNS in the human body. The brain and spinal cord work together to process and transmit information, coordinate body functions, and regulate responses to stimuli.