I believe the correct answer is the first option. The part of a vertebrate's brain that is the center for thinking and complex behaviors is the spinal cord. It is a system that is characteristic in vertebrates. It extends from the base of the brain though the spinal column. It has nerve fibers that are responsible for mediating reflex actions and the transmission of impulses to the brain and also from the brain. It is a part of the central nervous system that functions to supply nerves and in receiving the information from the PNS or the peripheral nervous system in the body.