After standing up abruptly, a person faints because the brain is short of blood. Raising the legs helps the blood flow to the brain. Sitting them up would hinder the blood flow, and slapping them or splashing water on them will not help.
Raising the legs is the correct first aid for fainting after standing up abruptly. With the help of gravity, it lets blood flow back from the lower body to the heart and brain, quickly restoring consciousness. Standing up abruptly after squatting for a long time makes blood collect in the legs, so the blood pressure in the brain and its oxygen supply drop for a while.
Option 1. Sitting them up hinders blood flow to the brain, because the head stays high, and this prolongs the unconsciousness.
Option 3. Splashing water does not solve the drop in blood pressure and can put an unconscious person at risk of choking.
Option 4. Slapping is harmful, causes pain and does nothing at all to restore blood flow to the brain.