Which AWS service will meet these requirements?

Explanations:

Amazon GuardDuty is a threat detection service that monitors for malicious activity and unauthorized behavior in AWS accounts and workloads, but it does not specifically monitor S3 buckets for PII.

Amazon Detective is used for security investigation and analysis, helping users to understand the details of potential security issues. It does not monitor S3 for PII.

Amazon Macie is designed specifically to identify, classify, and protect sensitive data stored in S3 buckets, including PII. It provides alerts about vulnerabilities related to PII.

AWS Shield is a managed DDoS protection service. It protects applications from DDoS attacks but does not monitor S3 buckets for sensitive information like PII.