Which developmental milestone should a 9-month-old child typically show?

At 9 months old, a child is typically engaging in canonical babbling, which is characterized by the repeated vowel-consonant combinations, such as "ba-ba" or "da-da." This stage marks an important phase in language development as it indicates the child is beginning to experiment with sounds and explore their vocal abilities.

Canonical babbling serves several key functions: it helps infants practice the mouth movements needed for later speech, allows them to gain social attention from caregivers, and sets the groundwork for speech development as they move towards forming meaningful words and phrases. By this age, children are also starting to understand rhythm and intonation patterns, which are crucial for future language skills.

The other options, such as producing two-word phrases, forming complex sentences, or following simple commands, typically develop later. For instance, two-word phrases generally emerge around 18 to 24 months, complex sentences appear later in early childhood, and the ability to follow simple commands can start developing around 12 months but is more pronounced as the child grows older. Thus, canonical babbling is the milestone accurately reflective of a 9-month-old child's typical development stage.