Which of the following behaviours characterize the 'concrete operational stage' as proposed by Jean Piaget ?

The correct option is: Conservation; class inclusion.

According to Jean Piaget's theory of cognitive development, children progress through four distinct stages: sensorimotor, preoperational, concrete operational, and formal operational. The concrete operational stage typically occurs between the ages of 7 and 11 years.

Key characteristics of the concrete operational stage include:
1. Conservation: The understanding that certain physical properties of objects (such as volume, mass, number, or quantity) remain unchanged even when their outer appearance or spatial arrangement is altered.
2. Class Inclusion (Hierarchical Classification): The ability to categorize objects into broader groups and sub-categories simultaneously, recognizing relations between part and whole (e.g., understanding that a group of 8 dogs and 2 cats contains 10 animals, so there are more animals than dogs).
3. Reversibility: The ability to mentally reverse a sequence of events or operations.

Why other options are incorrect:
- Hypothetico-deduction reasoning and propositional thought are features of the formal operational stage (ages 11 and up).
- Deferred imitation and object permanence are major milestones of the sensorimotor stage (birth to 2 years).
- Make-believe play and irreversibility of thought are characteristic features of the preoperational stage (ages 2 to 7 years).