When someone faints, the brain does not get enough blood. By laying the victim down and raising their legs, you help blood flow to the head, and they regain consciousness sooner.
Fainting is caused by a temporary drop in blood flow to the brain. To restore normal circulation, the victim must be laid on their back with their legs raised. This position lets blood flow easily from the lower parts of the body to the heart and brain, helping the victim regain consciousness quickly.
Option 1. A half-sitting position keeps blood from flowing up to the brain against gravity.
Option 3. Bending the legs is not enough to raise them, so blood flow to the head does not improve.
Option 4. Raising the head worsens the blood supply to the brain, which is exactly what causes fainting.