Final answer:
The nurse's first action should be to closely monitor the client's vital signs, as the extremely high blood pressure and fever indicate potentially life-threatening conditions that need immediate attention and could guide further treatment.
Step-by-step explanation:
The nurse should prioritize care based on the principles of triage, which dictate addressing the most life-threatening conditions first. In this scenario, the client's blood pressure of 210/105 mm Hg and temperature of 39.9 C (103.8 F) suggest a hypertensive emergency and possible severe infection or other medical conditions leading to high fever, both of which can pose immediate risks to the client's health.
Therefore, the correct first action for the nurse would be to C) Monitor vital signs closely, as it is key in detecting any rapid changes that may require urgent intervention, and it guides further management including the need for drug administration or checking for allergies or drug interactions.