Step 1: Understanding the Question:
The question relates to Jean Piaget's stages of cognitive development and asks which stage features the realization that objects exist even when hidden (Object Permanence).
Step 2: Detailed Explanation:
Piaget defined four stages of development:
1. Sensorimotor Stage (Birth to 2 years): Children learn about the world through senses and motor actions. The key milestone here is Object Permanence (usually developed around 8-9 months).
2. Preoperational Stage (2 to 7 years): Symbolic thought and language develop, but logic is missing (egocentrism).
3. Concrete Operational Stage (7 to 11 years): Logical thought about physical objects develops (conservation).
4. Formal Operational Stage (12 years and up): Abstract and hypothetical reasoning develops.
Step 3: Final Answer:
Object permanence is a milestone of the Sensorimotor Stage.