Which Option Represents the Upper Limit of Vygotsky's Zone of Proximal

Which option represents the upper limit of Vygotsky's zone of proximal development?

A) Barbara is teaching her cat to use the toilet instead of a litter box.
B) Micha volunteers with his Boy Scout troop to clean up litter on the side of the road.
C) Erin has learned to swim on her own and no longer wants her parents' help in the pool.
D) Sydney cannot currently finish the problems she has been assigned in algebra, but with her teacher's help she is able to get through the task.