What is the primary energy source?

• A. Proteins
• B. Lipids
• C. Water
• D. Carbohydrates

Answer: (D)

Carbohydrates are the body's main, readily available fuel because they are quickly converted to glucose, which powers most cellular activities and is especially important for the brain and red blood cells that rely on glucose for energy. Fats provide a dense energy source but must be mobilized and metabolized more slowly, making them the preferred fuel during longer, lower‑intensity activity rather than during immediate energy needs. Proteins are primarily used for growth and tissue maintenance and are not the body's first choice for energy; using them for energy is less efficient. Water supplies no calories, so it cannot serve as an energy source. Therefore carbohydrates are the best answer.