Regarding the "configure your web site’s Auto scaling group to use spot instances rather than on demand instances" answer, I can put a spot price the same as a on-demand price in the "Maximum price" field of the Launch Configuration. Doing so I will keep the spot instances running.
Regarding the "consolidate many smaller RDS instances into a single larger RDS instance" answer we learnd that it’s sometimes cheaper to have a large instance type instead of many smaller types. Despite the savings are not sure, this is a valid option.
I am also concerned about such things in the exam. The answer is theoretically correct, but others have another opinion on that. Would it be a fail on such questions?
Yes, some things items like these in the real world can fall down on on either side, but on the exam there will always be at least one piece of information that makes a single answer correct. For example, I recently put an analytics solution in place for a customer and during the proof of concept phase we were trying to determine what instance size would be ideal for the customer. We ran several of their largest queries a discovered that using a much larger sized instance ran the queries so much faster than a medium sized instance that they actually saved money by going larger. On the exam, there will be some information like average query execution time or the like that will drive you toward a correct architecture and sizing to ensure its not a matter of opinion.