Fainting occurs when there is a temporary reduction of blood flow to the brain. if a child faints, your immediate action would be to:

The answer to this question would be: c. gently lower the child to the floor

By putting the child to the floor, the child will be on lie down position. In this position, the head and the heart height would be same. This will make pumping blood into the brain easier since it won't fight the gravity force.
But the ideal choice would be to lie down the child so the height of his/her head will be same as the heart, then elevate the feet so the blood flow to the brain will be much higher.