Answer:

To enter the respiratory pathway and be used as a source of energy, fats need to be broken down into glycerol and fatty acids. This process occurs through the process of lipolysis.

1. Carbohydrates:
Carbohydrates are broken down into glucose molecules through the process of glycolysis. Glucose then enters the respiratory pathway and undergoes further oxidation in the citric acid cycle (also known as the Krebs cycle) to produce ATP.

2. Fats:
Fats, also known as triglycerides, are composed of glycerol and fatty acids. Before entering the respiratory pathway, they need to be broken down into these components. This process occurs in two steps: lipolysis and beta-oxidation.
- Lipolysis: Triglycerides are broken down into glycerol and fatty acids by the enzyme lipase. Glycerol can then be converted into glyceraldehyde-3-phosphate, which enters the glycolysis pathway.
- Beta-oxidation: Fatty acids are broken down into acetyl-CoA molecules through a series of reactions known as beta-oxidation. Acetyl-CoA then enters the citric acid cycle to produce ATP.

3. Proteins:
Proteins are composed of amino acids. Before entering the respiratory pathway, proteins need to be broken down into amino acids through the process of proteolysis.
- Proteolysis: Proteins are broken down into amino acids by the enzyme protease. Amino acids can then be converted into various metabolic intermediates depending on their structure. For example, some amino acids can be converted into pyruvate or intermediates of the citric acid cycle, which can enter the respiratory pathway and be used for energy production.

Conclusion:
Based on the information above, fats (option B) and proteins (option C) need to be broken down into their respective components before entering the respiratory pathway. Carbohydrates (option A) can directly enter the pathway after being broken down into glucose molecules. Therefore, the correct answer is option C, Only B and C.

Fats would need to be broken down into glycerol and fatty acids first. If fatty acids were to be respired they would first be degraded to acetyl CoA and enter the pathway. Glycerol would enter the pathway after being converted to Phosphoglyceraldehyde. Fatty acids are broken down to acetyl-CoA by means of beta oxidation inside the mitochondria.