Ref the question: ‘Based on past statistics of our web traffic, we observe that we sometimes get traffic spikes on Monday morning. What is the most cost-effective type of scaling should we use for this scenario?’
Dynamic is the suggested answer. However, given the specific nature of our analysis (weekly peaks), we run the risk of being under-provisioned for a sharp volume spike with dynamic scaling. Dynamic scaling takes ~5-10 minutes to respond.
Yes, being under-provisioned is a risk we take presuming the traffic spikes. The question asked which is the most cost-effective scaling. If the question also asked most cost-effective to ensure we would never be under-provisioned for 5 minutes or less, then that would change the answer. Again, you need to consider the question in an isolated box. Yes, this isn’t real world but neither is the exam and that’s what we’re training people to pass.