The correct answer to the Prep to Shoulder Sit quiz is the transition is Illegal in Level 1 as performed. The back starts as the spotter, but once she moves under the top person she no longer meets the definition of spotter. The transition then becomes Illegal due to being a prep level stunt without a spotter.
Here’s an example of a Legal version, in which the top person is lowered to waist level before the back is under the top person.
The explanation is based on the 2017-18 version of the cheer rules dated August 30, 2017.