Final answer:
The medulla oblongata is the part of the brain responsible for controlling vital functions such as breathing, heartbeat, and blood vessel size, making it the correct answer to the question.
Step-by-step explanation:
The part of the brain that controls breathing, heartbeat, and the size of blood vessels is the medulla oblongata.
The medulla oblongata is critical for regulating vital body functions. It is part of the brain stem, which also includes the midbrain and the pons. The medulla houses the control centers that manage cardiovascular and respiratory systems, ensuring that our heart beats and we breathe automatically without conscious effort.
Additionally, the medulla oblongata helps regulate other autonomic functions like blood pressure and digestion, and it serves as a pathway for motor and sensory neurons to relay signals between the brain and the spinal cord.