Realizing that rattles make a noise, a 4-month-old infant laughs with delight when his mother puts a rattle in his hand. this infant is displaying a reaction.

secondary circular

Explanation:

Secondary circular reaction: In psychology, this is one of the sub-stages of the sensorimotor stage which is a part of the cognitive development theory and it was given or introduced by Jean Piaget.

The secondary circular reaction tends to start from four months of age in an individual's life and lasts through eight months of age. In this sub-stage, a child tends to become focused towards the world or environment around him or her and therefore he or she tends to repeat few things just to trigger a particular response in the surrounding environment.

The given statement of the question above states that the infant is displaying a secondary circular reaction.