In his theory of cognitive development, Jean Piaget explains cognitive structures in terms of _________.

The correct option is Schemas.

Detailed Explanation:
In his pioneering theory of cognitive development, the Swiss psychologist Jean Piaget proposed that children actively construct their understanding of the world. He introduced the concept of schemas (singular: schema) to describe the basic cognitive structures or building blocks of knowledge. A schema is a mental framework, category, or representation that helps an individual organize, interpret, and make sense of incoming information and experiences.

According to Piaget, as children interact with their environment, they continuously adapt their schemas through two main complementary processes:
1. Assimilation: Fitting new information into existing schemas.
2. Accommodation: Modifying existing schemas or creating new ones because new information does not fit into what is already known.

Analysis of Other Options:
• Psychological tools: This concept is associated with Lev Vygotsky's sociocultural theory of cognitive development, referring to culturally determined tools (like language, counting systems, and writing) that mediate cognitive functioning.
• Stimulus-response association: This is a core concept of Behaviorism (supported by theorists like B.F. Skinner and Ivan Pavlov), which explains behavior and learning through external associations rather than internal mental structures.
• Zone of proximal development (ZPD): This is another key concept from Lev Vygotsky, defined as the range of tasks that a child cannot yet perform independently but can accomplish with the guidance and encouragement of a more knowledgeable helper.