After drinking alcohol, a person walks clumsily because the alcohol affects his cerebellum. The cerebellum is the part of the brain responsible for coordinating movement and maintaining balance. It receives signals from the sensory systems, the spinal cord, and other parts of the brain and uses this information to regulate motor movements.

When alcohol is consumed, it affects the functions of the cerebellum by interfering with the communication between neurons. It slows down the transmission of signals and disrupts the coordination and control of movements, leading to clumsiness and difficulty in walking properly.

The cerebrum, on the other hand, is responsible for higher-order functions such as thinking, reasoning, and decision-making. The medulla oblongata is a part of the brainstem involved in controlling vital functions such as breathing and heart rate. The spinal cord is a long, cylindrical structure that relays messages between the brain and the rest of the body.

In this case, the correct answer is the cerebellum because alcohol`s impact primarily affects the coordination of movement and balance, which are regulated by this brain region.