Which AWS service uses machine learning to help discover, monitor, and protect sensitive data that is stored in Amazon S3 buckets?

Explanations:

AWS Shield is primarily focused on DDoS protection for applications and does not involve machine learning for data discovery in S3.

Amazon Macie uses machine learning to discover, classify, and protect sensitive data stored in Amazon S3 buckets.

AWS Network Firewall is designed to protect VPCs with stateful, managed firewall capabilities, not for data monitoring in S3.

Amazon Cognito is a service for user identity and access management, and does not relate to data discovery or protection in S3.