The correct answer to the Flat Backs quiz is Flat Backs C and D are Legal in Level 1. The main difference in the Flat Backs pictured is the placement of the spotter. In Flat Backs C and D the spotter is in a Legal spotter position by not being under the top person, yet close enough to touch the stunt. Please remember the spotter is not required to touch the stunt, but must be close enough to.
In Flat Backs A and B below, the athlete on the head side of the top person is underneath the top person. This makes it so no athlete in the image meets the definition of spotter, which among other things says:
cannot stand so that their torso is under a stunt.
Level 1 requires each top person in a prep level stunt to have a spotter so both Flat Back A and Flat Back B are Illegal in Level 1.
The explanation is based on the 2017-18 version of the cheer rules dated August 30, 2017.