According to Vygotsky, What Is the Zone of Proximal Development

According to Vygotsky, what is the zone of proximal development?

A) Range of tasks too difficult for children to master either alone or with assistance
B) Range of tasks too difficult for children to master alone, but which can be learned with assistance
C) Range of tasks that children have mastered, but cannot teach to others
D) Range of tasks that children have mastered, and that they can teach to others