The diagram shows a sagittal section of the human brain with four regions labeled A, B, C, and D.

An individual is diagnosed with a slow-growing brain tumor that exerts physical pressure on the brain region located immediately superior to the pituitary gland. As a result, the individual exhibits symptoms of impaired thermoregulation, chronic sleep disruptions, and severe polydipsia (excessive thirst) due to disrupted osmoregulatory control.
Which lettered region on the diagram represents the structure whose function is directly compromised by this tumor?
Region A
Region B
Region C
Region D