Final answer:
For a 5-year-old with bradycardia, the nurse would first provide oxygen at 100%. This supports the cardiovascular system and is an initial step before additional interventions. The correct option is A.
Step-by-step explanation:
The first action a nurse would take for a 5-year-old girl with sustained bradycardia is A) Provide oxygen at 100%. Bradycardia refers to a slower than normal heart rate, and the immediate provision of oxygen can help support the child's cardiovascular system while the underlying cause of the bradycardia is determined and treated.
Options B, C, and D may be relevant depending on the situation, but securing the airway and providing adequate oxygenation is universally recognized as the initial step in pediatric resuscitation.