Final answer:
The nurse referred to the developmental milestones theory, indicating that children generally follow a specific timeline for reaching developmental stages such as sitting, which is influenced by both innate patterns and cultural practices.
Step-by-step explanation:
The nurse based the response to the client on the theory of developmental milestones, suggesting that there are specific normative events, such as sitting, that most children reach at approximate ages. This concept aligns with the view of discontinuous development, which posits that development occurs in distinct stages that unfold at certain times or ages throughout a child's growth. As such, while cultural practices and individual differences may influence the exact timing, the progression through developmental milestones generally follows an innate timetable that is similar across different children.