Which of the following is NOT one of the four major parts of the brain?

The cortex, while an important part of brain anatomy, is not classified as one of the four major parts of the brain. The four major parts are the cerebrum, cerebellum, brain stem, and diencephalon. The cerebrum is the largest part, responsible for higher brain functions like thought and action. The cerebellum is located under the cerebrum and is essential for coordination and balance. The brain stem connects the brain to the spinal cord and controls basic life functions such as breathing and heart rate. The cortex refers to the outer layer of the cerebrum and is involved in critical cognitive functions, but it is not one of the major structural divisions of the brain itself.