Answer: Vygotsky's
Explanation:
Vygotsky's theory states about development of cognition that takes place through social interactions. He believed that child development can be driven through socio-cultural aspects,language,psychological tools,proximal development zone etc.
According to the question, Mr. Pietrzyk's thinking about children;s development and learning through social interaction is depicting Vygotsky's theory.This helps them in peer meeting and groupings.