Final answer:
The three major categories used by researchers to organize the brain are the cerebral cortex, limbic system, and brainstem, each with its own specific functions.
Step-by-step explanation:
The three major categories researchers use to organize the entire brain are A) Cerebral cortex, limbic system, and brainstem. These categories represent different functional and structural parts of the brain with distinct roles. The cerebral cortex is involved in higher brain functions such as thought, memory, and personality, the limbic system is associated with emotions and memories, and the brainstem controls basic life functions such as breathing, heartbeat, and sleep patterns.