The correct answer to the L3 Full Down quiz is Illegal Stunt, specifically per Stunts A. Once the back grabs under the top person’s foot and becomes the only person under that foot, he is considered a base. This leaves the stunt above prep level without a spotter.
Another common variation of this skill is also illegal. In this video the back grabs the ankle of the top person, versus under the foot as done in the first video, which doesn’t change the stunt to a 2 leg stunt. This variation is illegal due to fulling down from a 1 leg stunt per Dismounts D.