Which service will meet these requirements?

Explanations:

AWS Trusted Advisor provides best practices and recommendations for optimizing AWS accounts but does not continuously monitor for suspicious activity or automate responses.

Amazon Detective helps investigate and analyze security issues and suspicious activities but does not actively monitor or take automated actions against threats.

Amazon Inspector is focused on assessing the security of applications and identifying vulnerabilities but lacks continuous monitoring capabilities for threats or automated responses.

Amazon GuardDuty is a threat detection service that continuously monitors for malicious activity and unauthorized behavior in AWS accounts. It can initiate automated actions based on security findings through AWS Lambda and other integrations.