Which child should the nurse refer for further hearing evaluation?

The child who is not making babbling sounds at 8 months old should be referred for further hearing evaluation because babbling is an important developmental milestone in speech and language acquisition. By 6 months of age, infants typically begin to babble, producing repetitive consonant-vowel combinations such as "ba-ba" or "da-da." The absence of babbling by 8 months may suggest a potential hearing impairment or other communication disorders, as this sound production is crucial for later language development.

In contrast, unintelligible speech at 18 months and a preference for social play in a 4-year-old are not necessarily indicators of hearing issues. Many toddlers at 18 months are still developing their speech, and varied levels of intelligibility can be expected. As for the 3-month-old infant with an exaggerated startle response, while this may indicate heightened sensitivity to sound, it does not definitively signal a hearing issue requiring evaluation at this age. Therefore, the lack of babbling in the 8-month-old is the most concerning sign that prompts a referral for further assessment.