Final answer:
B).The most cost-effective solution for handling the application's traffic pattern is to run four on-demand instances at all times and add eight additional on-demand instances on the last day of the month to accommodate the traffic spike.
Step-by-step explanation:
The most cost-effective way to handle the described web application traffic pattern is option b: running four on-demand instances constantly and adding eight more on-demand instances on the last day of each month. This approach is based on the assumption that the traffic is steady most of the time but spikes significantly on one specific day, which in this case is the last day of the month.
By using on-demand instances, the web application can adapt to the flexible workload without incurring the costs of running extra instances when they are not needed.
Reserved instances could be considered cost-effective if the higher traffic volume was constant or predictable over a longer period, justifying the upfront commitment. However, since the traffic triple only occurs once a month, utilizing additional on-demand instances only when needed is a more efficient use of resources. It also allows for greater flexibility in case the traffic patterns change in the future.