The respiratory structure that controls breathing is the diaphragm. It is the primary muscle responsible for breathing by contracting and relaxing to change the volume of the thoracic cavity, which allows air to flow into and out of the lungs. When the diaphragm contracts, it moves downward, increasing chest cavity volume and pulling air into the lungs; when it relaxes, it moves upward, pushing air out
. While the lungs are essential for gas exchange, they do not control the act of breathing itself. The diaphragm's rhythmic contractions, coordinated by neural signals from the brainstem, regulate the breathing process
. Therefore, the correct answer is: a. diaphragm
