Which solution will meet these requirements with the LEAST operational overhead?

Explanations:

While creating a custom metric and using CloudTrail for alarms is a viable method, it requires more setup and operational overhead to implement compared to other options. CloudTrail is primarily used for tracking API calls and changes in AWS resources, which may not directly relate to application-specific errors.

This option involves using an AWS Lambda function that scans CloudWatch logs every 5 minutes. While it would monitor for errors, the 5-minute interval introduces latency in alerting and requires continuous running of the Lambda function, leading to higher operational overhead.

This option is the most efficient as it utilizes CloudWatch Logs to create a metric filter for the specific error message. This setup allows real-time monitoring with minimal operational overhead. Once the metric filter detects the DECRYP_ERROR, a CloudWatch alarm can be triggered to send immediate notifications via SNS, fulfilling the requirements effectively.

Installing the CloudWatch unified agent adds unnecessary complexity to the solution. It involves more operational overhead by requiring additional configuration for the agent and metrics generation, while the existing capabilities of CloudWatch Logs and alarms would suffice to monitor for errors efficiently.