Which of the following is a red flag for a one-month-old's development?

A one-month-old not being able to focus with their eyes is a significant red flag for development. At this age, infants are expected to have some visual acuity and can typically follow movements with their eyes, particularly focusing on nearby faces. The ability to look at objects and caregivers is crucial for early social interaction and cognitive development. If an infant does not exhibit this eye focus, it may suggest underlying issues with vision or neurological development that warrant further evaluation.

The other developmental milestones mentioned are appropriate for a one-month-old. Smiling at caregivers is a positive social response that typically begins around this age. The ability to lift the head briefly during tummy time shows developing neck strength, and reacting to sounds indicates engagement with the environment and developing hearing abilities, all of which are expected at this stage of development.