The correct answer to the L6 Flip to Prone quiz is Illegal. The most correct rule to cite for this skill is Stunts D, specifically because of the 1st clarification, stating:
Clarification: Free flipping stunts and transitions that do not start on the performing surface are not allowed.
The skill does not meet the definition of dismount because it does not land in a cradle or on the performance surface. The skill meets the definition of a toss, but Tosses B says tosses must land in cradle position, which does not include landing prone.