Correct Answer:
D. Glucose
The correct answer is Glucose. In cellular respiration, glucose (C₆H₁₂O₆) is the primary source of energy. It is broken down through glycolysis, the Krebs cycle, and the electron transport chain to produce ATP, the main energy currency of the cell. Among the options:
- Protein: Used as an energy source only when carbohydrates and fats are unavailable.
- Lipids: Also used for energy, but glucose is the primary and preferred substrate.
- Vitamin C: An essential nutrient, not an energy source.
- Glucose: Correct; it is the main energy source for cellular respiration.
Therefore, the correct answer is "Glucose".