What Is the Zone of Proximal Development (ZDP)

What is the zone of proximal development (ZDP) ?

A) the amount of help that a student needs in order to experience maximal growth
B) the level of work that a student can do when working on his or her own
C) the difference between what a student can do on his or her own and what the student can do with help
D) the amount of growth that a student should attain in a year?s time