Theories, Processes, and Stages of Language Acquisition - California Teacher of English Learners (CTEL)

Social interactionist theory has its roots in the work of Lev Vygotsky, who focused on the role of linguistically fluent adults in the development of language in young children. As it applies to education, social interactionist theory argues for a direct, personally engaged role for primary teachers in the classroom. According to Vygotsky, direct and involved social interaction with a verbally acute adult lays the groundwork for verbal fluency and intelligence in young children.

While the other answers all named legitimate social theorists, none of their work applied to the social interactionist theory of language development specifically.