Trusted Advisor reports that all application servers for a financial service are flagged as Low Utilization EC2 instances. The application runs on three instances across three Availability Zones. The administrator needs to lower running costs without altering the application’s availability or architecture. Which action meets this requirement?

The correct answer is to follow rightsizing recommendations from AWS Cost Explorer to move to smaller instance types. Trusted Advisor flagged the instances as "Low Utilization," indicating they are over-provisioned for their current workload. Rightsizing to smaller, more cost-effective instance types directly addresses the goal of lowering costs without impacting availability or requiring architectural changes, as the application will still run across three Availability Zones. Reducing the number of application server instances would compromise availability, as the question specifies maintaining the current architecture and availability. Putting an Application Load Balancer in front of the servers is an architectural change and doesn't directly reduce costs associated with underutilized instances; it's more for traffic distribution and high availability. Increasing instance sizes would increase costs, not lower them, and is contrary to the "Low Utilization" finding.