According to vygotsky, if one child receives assistance he/she may improve substantially more than another child who does not receive . the assistance or structuring provided by others is termed

According to Vygotsky, the level at which a child cannot fully perform a task independently, but can do so with the assistance of someone more competent would be the zone of proximal development or ZPD. It would refer to the difference of a learner's capability of performing a task without any assistance and one with assistance.  

Ibelieve that the answer is (a).