When a person drinks alcohol and experiences difficulty walking or becomes clumsy, it is primarily due to the effect of alcohol on the cerebellum.

The cerebellum is a region of the brain that is responsible for coordinating voluntary movements, balance, and posture. It receives information from various parts of the body, including the sensory systems, and integrates this information to control and modulate motor movements.

Alcohol affects the cerebellum by depressing its activity. It interferes with the communication between the cerebellum and other parts of the brain, disrupting the normal functioning of the cerebellum. As a result, the person may have difficulty with motor coordination and balance, leading to a clumsy gait or stumbling while walking.

It is important to note that while alcohol primarily affects the cerebellum, it can also have an impact on other parts of the brain, including the cerebrum, pons, and medulla oblongata. However, these effects are not primarily responsible for the motor coordination issues observed when someone is intoxicated. Therefore, option 2, the cerebellum, is the correct answer.