Answer: The correct answer for the fill in the blank is Fat because they store twice as much energy per gram.
There are three sources of energy in animals that is proteins, fats, and carbohydrates. The most concentrated form of metabolic energy found in nature is fat. One gram of fat provides approximately 9 calories, whereas carbohydrates or proteins provide only 4 calories per gram.
Thus, most of the excess energy is stored as fats in animals.