If the volume of CO_2 evolved during respiration is less than the...

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

$RQ = \dfrac{\text{Volume of } CO_2 \text{ evolved}}{\text{Volume of } O_2 \text{ consumed}}$

According to the given condition, the volume of $CO_2$ evolved is less than the volume of $O_2$ consumed. This implies that the value of RQ is less than 1.

For carbohydrates such as glucose, fructose, and galactose, the volume of $CO_2$ evolved is equal to the volume of $O_2$ consumed, giving an RQ equal to 1.

For fats, the amount of oxygen in their molecules is relatively less compared to carbohydrates. Therefore, they require more external oxygen for complete oxidation, resulting in an RQ less than 1 (approximately 0.7).

Thus, the respiratory substrate must be fats.

Answer: fats