Answer: Tertiary circular reactions
Explanation:
Tertiary circulation reaction is one of the substages of Piaget's sensorimotor stage which is characterized by infants learning how to put a particular idea into effect. They (infants) use imagination or original ideas to create something new instead of using the same idea. They discover new ways of doing things by experimenting it. Thus, children in this substage fall between the ages of twelve months to eighteen months.
Dennis is in tertiary circular reactions of Piaget's sensorimotor substages.