According to Lev Vygotsky, cognitive development is guided by

According to Lev Vygotsky's sociocultural theory of cognitive development, language plays a fundamental role in shaping thought and structuring cognitive processes.

Vygotsky proposed that children initially use language as a social tool to communicate with others. Over time, this social speech is internalized and becomes self-directed speech, known as private speech. Children use private speech to guide, plan, and regulate their own behavior and thinking processes, especially when tackling difficult tasks. Eventually, this private speech becomes silent, internal dialogue or inner speech.

Let's evaluate the options to understand why this is the correct choice:
1. Processes of attention and perception: While important in cognitive psychology, these are lower-level mental functions that Vygotsky argued are transformed into higher mental functions primarily through social interaction and cultural tools (like language), rather than guiding development on their own.
2. Assimilation and accommodation that change ‘schemes’: This is the core concept of Jean Piaget's cognitive development theory, not Lev Vygotsky's.
3. Positive and negative reinforcement given by the environment: This concept belongs to Behaviorist theories (such as B.F. Skinner's operant conditioning), which focus on external stimuli and responses rather than internal cognitive construction.
4. Language in the form of private speech: This directly aligns with Vygotsky's view that language is the primary cultural tool that drives cognitive development and self-regulation.

Therefore, cognitive development is guided by language in the form of private speech.