Final answer:
According to Lev Vygotsky, thought and language initially develop independently but merge later during development due to sociocultural influences.
Step-by-step explanation:
Lev Vygotsky believed that thought and language develop independently at first but begin to merge as social and cultural mechanisms come into play. Specifically, Vygotsky's sociocultural theory of development highlights that a child's social world is crucial to the formation of language and thought. Therefore, the correct answer to the question about Vygotsky's belief on the development of thought and language is C. Thought and language develop independently at first and merge later in development.