Question

the aortic and carotid bodies are chemoreceptors that are most strongly sensitive to

blood o₂ level
blood plasma level
blood ph
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Explanation:

Aortic and carotid bodies are peripheral chemoreceptors. Their main function is to detect changes in blood oxygen levels. When blood oxygen ($O_2$) level drops (hypoxia), they send signals to the respiratory center in the brainstem to increase breathing rate and depth. Blood plasma level is mainly related to fluid balance regulation (e.g., by kidneys) and not the primary stimulus for these chemoreceptors. Blood pH changes can also affect chemoreceptors, but the aortic and carotid bodies are most strongly sensitive to $O_2$ levels among the given options.

Answer:

blood $O_2$ level