Which of the following is one of the three systems primarily involved in breathing?

The respiratory system is fundamentally responsible for the process of breathing. It includes the lungs and the airways, working together to facilitate the exchange of oxygen and carbon dioxide with the environment. This system is essential for providing the body with the oxygen it needs to function and for removing waste gases produced during metabolism.

While the nervous system plays a crucial role in controlling and regulating the breathing process, particularly through involuntary control centers in the brain that adjust breathing rate based on various factors, it is not one of the three primary systems involved specifically in the act of breathing. The circulatory system, while vital for transporting the oxygen absorbed by the respiratory system to the organs and tissues, does not directly perform the act of breathing itself. The digestive system is unrelated to respiration and focuses instead on breaking down food for nutrient absorption. Thus, the best answer highlighting the primary role in breathing is the respiratory system.