The primary cellular function of fatty acids is long term energy storage. The body stores small amount of excess nutrients as triglycerides for storage. Triglycerides are efficient energy storing molecules as more energy can be stored in fat than in glycogen. Fat contains 9 kcal per gram whereas carbohydrates and protein only contain 4 kcal per gram. In other words, fat is more calorie dense so can hold a greater amount of energy in a smaller space
So, the correct answer is 'Fat'