Which of the following is a developmental milestone expected in a 9-month-old infant?

At 9 months, infants typically reach several key developmental milestones, and one important social milestone is the ability to wave goodbye. This action indicates that the infant is beginning to understand social cues and engage in social interactions. Waving goodbye is a form of nonverbal communication that shows they are recognizing familiar people and responding to social contexts.

In contrast, walking independently is usually expected around 12 months of age. While some infants may take their first steps at 9 months, it is not the norm for this age group. Running with confidence is a milestone associated with toddlers who are generally around 2 to 3 years old. The pincer grasp, which involves using the thumb and forefinger to grasp small objects, typically develops between 9 to 12 months, so while it is a relevant milestone, it is not as specifically indicative of social interaction as waving goodbye. Therefore, the ability to wave indicates a more pronounced social development milestone that aligns with typical behavior for a 9-month-old infant.