What structure serves as a bundle of white matter linking the cerebral hemispheres?

The corpus callosum is the correct choice because it is the main structure in the brain composed of white matter that connects the left and right cerebral hemispheres. This bundle of nerve fibers facilitates communication between the two sides of the brain, allowing for the integration of sensory and motor information, as well as enabling higher cognitive functions.

The cerebral cortex refers to the outer layer of the brain and is involved in various high-level functions such as perception, thought, and decision-making, but it is not a connecting structure between the hemispheres. The thalamus acts as a relay station for sensory information heading to the cortex and is not primarily responsible for linking the hemispheres. The medulla is part of the brainstem that controls vital autonomic functions such as breathing and heart rate, and it does not serve the role of connecting hemispheres. Thus, the corpus callosum is specifically designed for interhemispheric communication, making it the correct answer.