A nurse is caring for an 18-month-old toddler who has been hospitalized for 10 days. After the toddler's mother leaves the room, the nurse observes the toddler sitting quietly in the corner of the crib, sucking her thumb. When the nurse approaches the crib, the toddler turns away from the nurse. The nurse should understand that these behaviors indicate which of the following developmental reactions?

A. Developing autonomy would involve behaviors like exploring the environment and showing independence, not withdrawing and sucking the thumb. B. Anxiety reactions are common in toddlers who are hospitalized or experience separation from their primary caregivers. It is a normal response to unfamiliar and potentially stressful situations. Toddlers at this age are still developing a sense of security and trust in their environment, and being in the hospital can disrupt their routine and comfort. C. While regression can occur due to stress, the specific behaviors described (sitting quietly, thumb-sucking) are more typical of regression related to developmental delays or changes in routine. D. Resentment toward the mother would be more complex and not typically seen in an 18-month-old who is still highly dependent on the primary caregiver.