Final answer:
The brain stem is identified by option A in the diagram. It connects the brain to the spinal cord and plays a crucial role in relaying signals, coordinating motor control, and controlling bodily functions.
Step-by-step explanation:
The brain stem is identified by option A in the diagram. It is the base portion of the brain that connects to the spinal cord. The brain stem consists of the midbrain, medulla oblongata, and the pons. It plays a crucial role in relaying signals between the brain and spinal cord, coordinating motor control signals, and controlling important bodily functions such as breathing, heart rate, and digestion.