One of the practice question had requirement of EC2 instance for a task which is once a month, non-negotiable interruption (cannot be interrupted) and needs aggressive pricing.
In my view the answer should be On Demand instances but the correct answer was Spot instance. that is where I was confused. Spot instance are vulnerable for interruption and in my view not being interrupted should take precedence over aggressive pricing.
Without additional context, I would agree with you. If you see the question again I would raise it as feedback.