Which of the following structures forms major parts of the brain?

The major parts of the brain include structures that play critical roles in its overall function and organization. The correct choice identifies the brain stem, cerebellum, diencephalon, and cerebrum as key components.

The brain stem serves as a vital relay station for information traveling between the brain and the spinal cord and is responsible for regulating essential functions such as breathing, heart rate, and consciousness. The cerebellum is crucial for coordinating voluntary movements, balance, and motor control. The diencephalon, comprising structures like the thalamus and hypothalamus, acts as a major relay and processing center for sensory information and is involved in various regulatory functions including temperature control and hormonal regulation. Lastly, the cerebrum is the largest part of the brain, responsible for higher brain functions such as thought, emotion, and voluntary muscle movements.

This combination of structures provides a comprehensive overview of the brain's functional capabilities, making this selection the most accurate representation of its major components. In contrast, other options either include structures that are not classified as major parts of the brain or fail to encompass the necessary components effectively.