Which of the following respiratory substrates yields a Respiratory...

The Respiratory Quotient (RQ) is defined as the ratio of the volume of $CO_2$ evolved to the volume of $O_2$ consumed during respiration.

For carbohydrates like glucose, the RQ is $1.0$.

For fats and fatty acids, the RQ is less than $1$, typically around $0.7$.

For proteins, the RQ is approximately $0.9$.

For organic acids like malic acid, the RQ is greater than $1$ (approximately $1.33$).

Therefore, proteins yield a Respiratory Quotient of approximately $0.9$.

Answer: Proteins