Final answer:
Lev Vygotsky is the theorist who believed that a child's cognitive development was most affected by sociocultural influences, emphasizing the role of culture and social interaction in shaping language and thought.
Step-by-step explanation:
The theorist who believed that a child's cognitive development was most affected by sociocultural influences was Lev Vygotsky. Vygotsky proposed a sociocultural theory of development, emphasizing that human development, particularly the formation of language and thought, is deeply rooted in a person's culture and social interactions. He highlighted the significance of one's cultural background and historical influences on cognitive development and how individuals interact with their environment to shape their learning process.