None of the above. All of these are involved in regulating respiratory rate. Let’s say you went out for a run. As you pump your legs, the muscles increase their activity at the cellular level, using up more oxygen and producing more CO2 as their metabolic activity speeds up. But at this point your lungs are still breathing at the normal rate,and CO2 quickly builds up in your bloodstream. This lowers the pH (dissolved CO2 makes the blood more acidic).
Your brain gets a signal saying ‘pH is too low!’ and that signal is processed by the pons and medulla oblongata, which then sends another signal to the lungs. The signal directs them to increase the rate of breathing so that they can keep up with your muscles’ increased demand for oxygen and increased output of CO2.