Estar is a temporary condition so why is the answer ser regarding being sick?
I can't find any question that relates the verb "ser" to being sick, but I've seen a question about professions that mentions nurses, though. Could it be possible that you've confused "enfermo" (sick) with "enfermera" (nurse)? Remember that when talking about professions, we always need the verb "ser" and not "estar".
These Kwiziq lessons will help you see in context both verbs ser and estar in these two situations:
Using ser to say what you do for a living
Using estar to express a physical condition
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