Final answer:
The respiratory center, responsible for controlling respiratory rate and depth, is located in the brainstem, specifically within the medulla oblongata and the pons. Therefore, the correct option is D.
Step-by-step explanation:
The respiratory center is located in the brainstem. The respiratory centers, which control both respiratory rate and depth, consist of the dorsal respiratory group (DRG) and the ventral respiratory group (VRG) found within the medulla oblongata, as well as the pontine respiratory group located within the pons, comprising the apneustic and pneumotaxic centers. The DRG and VRG are essential for maintaining a regular breathing rhythm and enabling forced breathing, respectively, and the pons centers regulate overall respiratory rate and depth of breathing.