AWS CodeDeploy natively supports automatic rollback based on CloudWatch alarms. To use this capability, you define an alarm that indicates unhealthy behavior (for example, increasing application error rate) and attach that alarm to the deployment group in CodeDeploy. If the alarm goes into ALARM state during or after a deployment, CodeDeploy automatically rolls back to the last known good revision when auto rollback is enabled.

Option A correctly implements this pattern:

Derive a CloudWatch metric from application logs (e.g., count of error-level log entries or HTTP 5xx responses).

Create a CloudWatch alarm on that metric with appropriate thresholds.

Configure the deployment group to use the alarm as part of its alarm configuration and enable auto rollback.

Option B uses CodeDeploy agent logs, which may not directly reflect true application health and couples rollback logic tightly to deployment internals rather than application behavior. Options C and D implement custom rollback logic with Lambda and EventBridge, which re-create behavior that CodeDeploy already provides natively and add unnecessary operational and code complexity.

Therefore, Option A is the simplest and most aligned with built-in CodeDeploy features for automated rollback based on application errors.