Final answer:
The pineal gland is the structure attached to the roof of the third ventricle. The third ventricle, located in the brain's center, is bordered by the thalamus and hypothalamus. The pineal gland is located inferior and posterior to the thalamus.
Step-by-step explanation:
Among the options provided, the structure that is attached to the roof of the third ventricle is the pineal gland. The third ventricle is a narrow cavity located in the middle of the brain, and the pineal gland is attached to its roof. The ventricle itself is bounded by the medial walls of the thalamus and hypothalamus. Conversely, the pituitary gland is attached to the hypothalamus at its base, rather than its roof. The hypothalamus forms the walls of the third ventricle but is not attached to the roof. Lastly, the corpus callosum is not associated with the third ventricle at all, as it is a large band of neural fibers that connects the two cerebral hemispheres.