Final answer:
The ratio of CO₂ produced to O₂ consumed is known as the respiratory quotient (RQ), an indicator of the type of macronutrient being metabolized for energy. The correct answer is d. respiratory quotient.
Step-by-step explanation:
The ratio of CO₂ produced relative to O₂ consumed is called the respiratory quotient (RQ). This ratio is an important parameter that provides insight into which type of fuel a body is metabolizing for energy.
Carbohydrates, for instance, yield an RQ of 1.0, implying an equal molar production of CO₂ and consumption of O₂. Other macronutrients such as fats and proteins have different RQ values, typically around 0.7 for fats and 0.8 for proteins, due to their different chemical structures and metabolic pathways.