Final answer:
The animal with a respiratory exchange ratio of 0.77 is most likely utilizing lipids as its main source of fuel, as evidenced by the typical RQ values for different macronutrients.
Step-by-step explanation:
The respiratory exchange ratio (RQ) is an important concept in biology that helps understand what type of fuel an animal is using during cellular respiration. The RQ is a dimensionless number that represents the ratio of carbon dioxide produced to oxygen consumed by an organism's metabolism. In the context of fuel sources, an RQ of 0.7 to 0.8 generally indicates lipid (fat) oxidation, while an RQ of around 1.0 indicates carbohydrate metabolism.
Given that the animal's respiratory exchange ratio is 0.77, we can infer that it is predominantly utilizing lipids as a fuel source at the moment. This is because the RQ for fat typically hovers around 0.7, whereas for proteins, it is about 0.8, and for carbohydrates, it is approximately 1.0. Therefore, the correct answer to the question is B. lipids.