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.