Step 1: Think about what "fuel" means for the body.
Fuel is whatever the body burns to release energy for its cells. Every gram of food we eat carries a certain number of calories, and the body picks which type to burn first.
Step 2: Rank the nutrients by how the body uses them.
Carbohydrate is stored as glycogen in the liver and muscles and gets broken down into glucose fastest. Fat is a backup fuel store used once carbohydrate runs low. Protein is kept mostly for building muscle, skin, enzymes and other tissue, and is only burned for energy in emergencies like starvation.
Step 3: Rule out the wrong choices.
Protein's real job is growth and repair, not routine fuel. Vitamins hold zero calories, so they cannot power anything on their own, they only help other reactions run smoothly. Water has no calories either, it just moves things around inside the body.
Step 4: Reach the answer.
Since carbohydrate is broken into glucose and burned first for daily energy needs, it is the body's true fuel.
The correct answer is Carbohydrate.