Answer:
The correct answer is carbon dioxide.
Explanation:
Respiration (physical) process involves inhalation and exhalation of oxygen and carbon dioxide respectively in between the air and into the lungs to the blood. If the brain receptors sense the excess amount of carbon dioxide in the blood, they send signals the body to promote ventilation. So, the most important chemical regulator of respiration is carbon dioxide that determines the respiratory rate.
An increase in carbon dioxide concentration in blood results in a decreasing pH level of the blood, causing the person to increase his rate of ventilation.
Thus, the correct answer is carbon dioxide.