Step 1: Understand the 'Zone of Proximal Development' (ZPD). ZPD is the difference between what a learner can do independently and what they can do with guidance or collaboration. Step 2: Identify the theorist associated with ZPD. Lev Vygotsky is the theorist who developed the concept of ZPD and emphasized social interaction in learning. Step 3: Conclude. The correct answer is (3), as Vygotsky proposed the ZPD and the concept of collaborative learning.