The human brain uses about 20% of the total oxygen and blood in the body. Although the brain weighs only about 2% of a person's body weight, it requires a large share of the body's energy and oxygen to function.

The brain controls every part of the body — thoughts, emotions, movement, senses, breathing, and more. All of this activity needs a constant and large energy supply. Oxygen is needed to produce energy in brain cells, and that oxygen is delivered through the bloodstream.

If the brain does not receive enough oxygen, brain cells begin to die within minutes. This is why problems like strokes (when blood flow to the brain is blocked) can cause serious damage or even death. Even while you are resting or sleeping, the brain is active and still needs oxygen.

Now, let’s consider the options:
- 12% is too low and does not match what science shows.
- 35% and 48% are too high for the average oxygen demand.
- 20% is the scientifically accepted answer. Most medical and science references agree on this percentage.

This tells us how important it is to keep our brain healthy. Regular exercise, good sleep, and healthy eating help make sure that the brain gets the oxygen it needs.

So, the correct answer is 20% — the brain uses 20% of the body's total oxygen and blood supply.