What is the MOST cost-effective way to manage these resources while maintaining fault tolerance?

Explanations:

Using an Auto Scaling group allows the company to automatically adjust the number of EC2 instances based on the predictable peak traffic load. This ensures that there are enough resources to handle the increased demand while also scaling down during low traffic periods, thus optimizing costs. Additionally, Auto Scaling groups can maintain fault tolerance by automatically replacing unhealthy instances.

Purchasing Reserved Instances may provide cost savings over on-demand instances if they are used consistently. However, since the EC2 instances are idle for the majority of the day, this approach would lead to unnecessary costs, as the company would be paying for capacity that is not being utilized.

While writing a cron job to stop EC2 instances during low traffic can reduce costs by not running idle instances, it may introduce delays when traffic demand suddenly increases, as the instances would need to be started again. This approach does not maintain fault tolerance because there would be no running instances during low traffic periods to handle any unexpected traffic spikes.

Writing a script to vertically scale EC2 instances during peak traffic demand can ensure resources are available during high traffic periods. However, vertical scaling has limitations, such as downtime during resizing and the risk of hitting instance type limits. Moreover, it does not address the cost-effectiveness for idle times, as instances would still need to be running at a larger size even when traffic is low.