Configure a cron job on each instance that publishes memory usage as a custom metric to CloudWatch is correct because a lightweight host script can read memory statistics on each Amazon Linux 2 instance and call the CloudWatch PutMetricData API on a schedule so that metrics are available for aggregation and alarms.

The Configure a cron job on each instance that publishes memory usage as a custom metric to CloudWatch approach scales to all 240 instances by running a simple scheduled script that samples memory from the OS and sends a custom metric. This lets you use CloudWatch dashboards and alarms and it keeps the solution lightweight and auditable. You can also use the CloudWatch Agent for the same goal if you prefer a supported agent instead of a custom script.

Use the default Amazon EC2 metrics in CloudWatch to obtain memory utilization is incorrect because the standard EC2 metrics do not include guest OS memory usage and you must publish a custom metric or use an agent to capture that data.

Query the instance metadata service to read live RAM usage from each instance is incorrect because the instance metadata service provides instance identity and configuration data and it does not expose dynamic performance counters like current memory utilization.

Enable AWS Systems Manager Inventory to gather memory utilization across all instances is incorrect because Inventory is designed for software and configuration inventory and it does not provide continuous real time memory metrics.

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