Final answer:
A tertiary circular reaction example for a child aged 12-18 months is intentionally dropping a toy to observe an adult's reaction, showcasing advanced cognitive and motor skills.
Step-by-step explanation:
An example of a tertiary circular reaction in the 12-18 month age range is when a toddler discovers a new way to achieve a goal with an object. For instance, the child might deliberately drop a toy over the side of a highchair to watch how an adult picks it up. This is a form of experimentation where the child is able to anticipate the outcome based on prior experience and is interested in observing the consequences of their actions. Such activities are indicative of the advancing cognitive and motor skills during this stage of development, aligning with Jean Piaget's theory of cognitive development.