Explanation:

Respiratory quotient (R.Q.) is a ratio that indicates the relative amounts of carbon dioxide produced and oxygen consumed during cellular respiration. It is calculated by dividing the volume of carbon dioxide produced by the volume of oxygen consumed. The R.Q. value can provide information about the type of respiratory substrate being utilized by the organism.

The R.Q. value can range from 0.7 to 1.0, depending on the type of substrate being used. Different substrates have different R.Q. values because they undergo different metabolic pathways and produce varying amounts of carbon dioxide and water.

Respiratory Substrates and R.Q. values:

1. Sucrose: Sucrose is a disaccharide composed of glucose and fructose. When sucrose is metabolized in the body, it undergoes glycolysis to be converted into glucose. The R.Q. value for glucose is 1.0, but during the breakdown of sucrose, some energy is lost in the conversion of fructose to glucose. Therefore, the R.Q. of sucrose is slightly less than 1.0.

2. Fat: Fats are a rich source of energy and are broken down through a process called beta-oxidation. The R.Q. value for fats is less than 1.0 because fat metabolism produces more carbon dioxide and water per mole of oxygen consumed compared to carbohydrate metabolism.

3. Glucose: Glucose is the primary respiratory substrate for most organisms. During glucose metabolism, it undergoes glycolysis, Krebs cycle, and oxidative phosphorylation to produce carbon dioxide and water. The R.Q. value for glucose is 1.0 because it produces equal amounts of carbon dioxide and oxygen.

4. Organic acid: Organic acids, such as acetic acid or lactic acid, can be used as respiratory substrates. The R.Q. value for organic acids is less than 1.0 because they produce more carbon dioxide and water per mole of oxygen consumed.

Conclusion:

In the given options, only fat has an R.Q. value less than 1.0. Fat metabolism produces more carbon dioxide and water per mole of oxygen consumed, resulting in an R.Q. value less than 1.0. Therefore, option 'B' (Fat) is the correct answer.

RQ is less than one when respiration is aerobic but the respiratory substrate is either fat or protein. RQ is about 0.7 for most of the common fats.