You inherited a GKE web application and need to check whether it uses libraries with known vulnerabilities or is vulnerable to XSS. Which service should you use?

Web Security Scanner is the correct choice because it specifically identifies vulnerabilities like XSS and outdated, vulnerable libraries in your web applications. It's designed to scan deployed applications for common web security flaws. Google Cloud Armor is a DDoS protection and WAF service; it protects against attacks but doesn't scan for application-level vulnerabilities. Cloud Debugger helps inspect the state of a running application to diagnose issues but isn't a security scanner. Error Reporting aggregates and analyzes errors from your applications, which is useful for operational health but not for proactive vulnerability scanning.