The company needs fast, proactive alerts for future performance issues, beyond basic EC2 metrics. Option A provides a complete, AWS-native monitoring pattern aligned with best practices:

Enable detailed monitoring on EC2 instances to increase metric resolution (from 5-minute to 1-minute intervals), improving the responsiveness of alarms.

Define custom CloudWatch metrics for application-level indicators such as request latency, error rate, queue depth, or throughput. These metrics can be published from the application or sidecar agents.

Create CloudWatch alarms on both infrastructure (CPU, network, disk) and custom application metrics with thresholds that reflect performance SLOs. Alarms can notify teams via SNS or incident management tools.

Use CloudWatch Logs Insights to analyze logs for recurring error patterns, slow requests, or exceptions when alarms fire.

Option B focuses on tracing and frontend RUM; while useful, it is more complex and not necessary just to get quick alerts. Option C uses services (CloudTrail, GuardDuty, Trusted Advisor) that are not focused on real-time performance detection. Option D with VPC Flow Logs is network-level and would not detect general application performance issues.

Thus, Option A offers a direct, efficient way to detect and alert on performance degradations quickly.