Developmental Milestones

Regarding developmental milestones in infants, all of the following are achieved by 12 months of age EXCEPT:

Which developmental milestone best distinguishes a 9-month-old infant from a 6-month-old infant?

During a routine well-child visit, a 3-year-old boy is assessed for gross motor milestones. The child can walk up stairs with alternating feet and jump with both feet together. Which is the most common cause of failure to achieve these milestones by age 3 years?

A 9-month-old male infant from Delhi is brought to the pediatric clinic by his mother with concerns about delayed development. On examination, the child cannot sit without support, does not transfer objects from one hand to the other, and does not babble or produce consonant sounds. He has normal tone and reflexes. Birth history is unremarkable with normal delivery and no perinatal complications. What is the most likely developmental concern in this infant?

A 12-month-old child is brought for developmental assessment. Which single finding most reliably distinguishes normal 12-month development from 9-month development?