Hey @Greg, you should always do some research about the company and what technologies and software they are using before going for the interview. Obviously, they will not just require one technology(DevOps). Its always a combination and the most famous combination is AWS and DevOps.
The answer to your question is A.
Since the work we are addressing here is not continuous, a reserved instance shall be idle at times, same goes with On-Demand instances. Also, it does not make sense to launch an On-Demand instance whenever work comes up since it is expensive. Hence Spot Instances will be the right fit because of their low rates and no long term commitments.
Also, have a look at this AWS interview questions article for more such questions: https://www.edureka.co/blog/interview-questions/top-aws-architect-interview-questions-2016/