Final answer:
According to Piaget, Erica is in the primary circular reactions substage of the sensorimotor stage.
Step-by-step explanation:
According to Piaget's theory of cognitive development, Erica, who is 3 months old and has learned to hold her head erect, is in the primary circular reactions substage of the sensorimotor stage. This stage lasts from birth to about 2 years old, and during this time, children learn about the world through their senses and motor behavior.
In the primary circular reactions substage, infants repeat actions that initially occurred by chance and are related to their own body movements. For example, Erica might accidentally touch her face and then repeat the action intentionally because she enjoys the sensation. This marks the beginning of purposeful behavior.
As Erica progresses through the sensorimotor stage, she will eventually move on to other substages such as secondary circular reactions and coordination of secondary circular reactions, which involve more complex interactions with objects and people.