Final answer:
Carbon dioxide levels in the blood is the factor that can override brainstem control of breathing in an infant, as it directly affects the central and peripheral chemoreceptors that regulate respiratory rate.
Step-by-step explanation:
The control of breathing in infants, as in adults, is mainly regulated by the brainstem, specifically the medulla oblongata and the pons. One of the primary factors that can override this control is the level of carbon dioxide (CO2) in the blood. Central and peripheral chemoreceptors monitor blood CO2 levels and signal the brainstem to adjust breathing as necessary to maintain homeostasis. Therefore, the factor among the listed options that can override brainstem control of breathing in an infant is carbon dioxide levels in the blood.