According to Vygotsky, What Does the Term Zone of Proximal

According to Vygotsky, what does the term zone of proximal development refer to?

A) tasks that are learned when children are placed together without adult supervision
B) tasks that are similar in the level of cognitive understanding required to successfully complete them
C) tasks that are completely mastered by the child in the previous level of cognitive understanding
D) tasks that are too complex for the child to complete alone but that can be accomplished with guidance and encouragement from a partner who is more skilled