Piaget is mainly known for studying:

Concept

Developmental theories explain systematic changes in children's abilities and behaviour over time.

Cognitive-developmental theory specifically examines how thinking, reasoning, and knowledge structures change through qualitatively different stages.

Application

Jean Piaget proposed the sensorimotor, preoperational, concrete operational, and formal operational stages. Each stage describes a characteristic way in which a child understands and reasons about the world, so the central subject of his work is cognitive development.

Contrast and cross-check

• Language development studies the acquisition and use of linguistic structures; it is not the principal classification of Piaget's stage theory.

• Sexual development is central to psychosexual frameworks, not to Piaget's account of changing reasoning structures.

• Social development focuses on relationships, roles, and social behaviour rather than the central structure of Piaget's stage model.

The names and defining abilities of Piaget's four stages all track changes in thought—from action-based understanding to abstract reasoning—which independently confirms the category.

Result

Therefore, Piaget is mainly known for studying cognitive development.