Fats are much more saturated than carbohydrates and more reduced, with more hydrogens instead of hydroxyl groups. This means that there is more energy stored in the bonds of fats, making them a better fuel.
Work Step by Step
To get to this answer, look at the structures of sample carbohydrates and fats in this chapter. Look for signs of reduction (single bonds, hydrogens rather than more oxidized functional groups bonded to carbon) to make this comparison. Keep in mind that reduction = more energy per carbon = better fuel.