Final answer:
Option (a), Increased pulmonary ventilation at high altitudes is primarily stimulated by chemoreceptors that sense the lower partial pressure of oxygen in the blood, leading to acclimatization.
Step-by-step explanation:
Increased pulmonary ventilation during acute exposure to altitude is stimulated primarily by chemoreceptors. These receptors are sensitive to levels of various substances in the blood, such as oxygen (O2), carbon dioxide (CO2), and hydrogen ions (H+).
In the context of altitude, where there is a lower partial pressure of oxygen, peripheral chemoreceptors located in the carotid bodies detect a decrease in the partial pressure of dissolved oxygen in the blood. This in turn stimulates an increase in the respiratory rate to help compensate for the lower availability of oxygen, a process known as acclimatization to high altitude.