**Core Concept**
The question is testing the understanding of developmental milestones in infants, specifically the age at which a child should be assessed for delayed development if they are not reaching for objects. Reaching for objects is a fundamental motor skill that typically develops between 4-7 months of age, with most children mastering it by around 6-7 months. If a child is not reaching for objects by 9 months, it may indicate a potential developmental issue that requires further assessment.

**Why the Correct Answer is Right**
The American Academy of Pediatrics (AAP) recommends that children who are not reaching for objects by 9 months of age should undergo further developmental assessment. This is because reaching for objects is a critical milestone that reflects cognitive, motor, and sensory integration skills. If a child is not achieving this milestone by 9 months, it may indicate a delay in one or more of these areas, which could be a sign of a developmental disorder such as cerebral palsy, developmental delay, or other neurodevelopmental disorders. The 9-month cutoff is chosen because it allows for early identification and intervention, which can significantly impact the child's long-term outcome.

**Why Each Wrong Option is Incorrect**
**Option A:** 3 months is too early to assess for delayed development, as most children are still in the process of developing their motor skills and may not have reached the stage of reaching for objects yet. By 3 months, children are typically still in the primary reflex stage and may not have the cognitive and motor abilities to reach for objects.
**Option B:** 5 months is still relatively early to assess for delayed development, as many children are still developing their motor skills and may not have fully mastered reaching for objects. While some children may be reaching for objects by 5 months, others may not, and a 5-month cutoff may not accurately identify children who need further assessment.
**Option D:** 12 months is too late to assess for delayed development, as most children have already developed the motor skills necessary to reach for objects by this age. By 12 months, children typically have a robust repertoire of motor skills, including reaching, grasping, and manipulating objects.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that developmental milestones are not a one-size-fits-all approach. Children develop at different rates, and some may need more time to master certain skills. However, if a child is not reaching for objects by 9 months, it's crucial to undergo further assessment to rule out any underlying developmental issues.

**✓ Correct Answer: C. 9 months**