What developmental milestone is indicated by a 12-month-old child searching for a toy that has been hidden?

A 12-month-old child searching for a toy that has been hidden demonstrates the developmental milestone of object permanence. This milestone occurs as children begin to understand that objects continue to exist even when they cannot be seen. By this age, infants typically start to actively look for items that are out of sight, indicating they have developed this cognitive skill. Object permanence is crucial as it reflects a child’s growing mental capabilities and ability to form mental representations of their surroundings.

The other choices represent different developmental aspects that do not apply directly to this behavior at the age of one. Imitation usually becomes more noticeable as children start to mimic actions they see in adults, separation anxiety typically manifests a bit later as children become aware of their caregivers' absence, and language development involves the gradual acquisition of communication skills rather than the understanding of object permanence. Thus, the correct answer effectively highlights this critical cognitive development in a child’s growth.