Final answer:
Caffeine primarily affects the central nervous system, particularly the brain's cerebral cortex and brainstem, by preventing drowsiness and stimulating parts of the autonomic nervous system.
Step-by-step explanation:
Caffeine is a central nervous system (CNS) stimulant and affects the brain by blocking the actions of adenosine on its receptors, preventing drowsiness, and stimulating parts of the autonomic nervous system. The CNS consists of the brain and the spinal cord. When considering the options provided—brainstem (A), cerebral cortex (B), peripheral nervous system (C), and skeletal muscles (D)—caffeine primarily affects parts of the brain, such as the cerebral cortex and brainstem, which are involved in the CNS. However, the peripheral nervous system (C) is indirectly affected through CNS functions.
The brain controls the CNS and regulates virtually all human activity. The brain can be divided into several parts, including the cerebrum or cortex, brainstem, and cerebellum. Given the information, the most accurate answer to the student's question, "Caffeine affects all parts of the brain, including the:", is (B) Cerebral cortex.