Final answer:
The nurse encourages the mother to be present to reduce the child's anxiety and ease respiratory efforts due to acute laryngotrachobronchitis.
Step-by-step explanation:
The rationale for the nurse encouraging the mother of a 20-month-old with acute laryngotrachobronchitis (LBT) to stay at the bedside as much as possible is C) The mother's presence will reduce anxiety and ease the child's respiratory efforts. At this vulnerable age, a child's sense of security is closely tied to the presence of primary caregivers, and stress or anxiety can exacerbate respiratory difficulties. The familiar and comforting presence of a parent can help to calm the child, which may facilitate easier breathing and improve clinical outcomes. Additionally, while the mother can indeed provide constant observation, which is beneficial, the primary reason in this context is the psychological comfort the child receives, which can have a direct impact on the physiological symptoms of LBT.