Final answer:
Breathing is primarily regulated by monitoring blood pH levels, directly influenced by carbon dioxide levels, hence the control of breathing is most tightly linked to pH.
Step-by-step explanation:
The control of breathing is primarily linked to blood pH levels, carbon dioxide levels (PCO₂), and, to a lesser extent, oxygen levels (PO₂). The respiratory system regulates the concentration of carbon dioxide in the blood by changing the rate and depth of breathing. When carbon dioxide levels rise, breathing accelerates to expel more CO₂, thus reducing carbonic acid and raising the pH towards normal levels. Conversely, if CO₂ levels are too low, breathing slows to retain more CO₂, which increases carbonic acid and lowers the pH. Therefore, the correct answer to the question is option 3) pH, as this is the most directly affected by the respiratory control of breathing.