Final answer:
The 13-month-old toddler unable to walk requires further follow-up, as most toddlers can stand and walk with help by their first birthday. This indicates a potential delay in physical development which needs to be assessed by a healthcare provider.
Step-by-step explanation:
The toddler requiring further follow-up is the 13-month-old unable to walk. By their first birthday, most toddlers can pull themselves up to a standing position and walk with help, if not alone. The motor skill development at that age includes being able to stand while holding onto furniture or someone's hand. A 13-month old should typically be able to stand and potentially take steps with assistance. If the toddler is unable to walk, this may indicate a delay in physical development and warrants further examination to determine the underlying cause and appropriate intervention.
In comparison, the 30-month-old drinking from a sippy cup, the 24-month-old crying during examination, and the 20-month-old using two- and three-word sentences are within normal bounds for their age groups in terms of development.