The correct choice is Amazon Macie because it is a fully managed service that discovers, classifies, and helps protect sensitive data such as personally identifiable information stored in Amazon S3 at scale.

Amazon Macie uses machine learning and pattern matching to identify PII and other sensitive data across many S3 buckets and it provides automated workflows and reporting to help secure that data. This makes it suitable for a 45 TB repository spread across more than 80 buckets where automated, scalable discovery and classification are required.

AWS Key Management Service (AWS KMS) manages encryption keys and controls access to those keys but it does not scan or classify S3 objects so it cannot discover PII by itself.

AWS Secrets Manager securely stores and rotates secrets such as database credentials and API keys and it is not designed to inspect stored data for sensitive content.

Amazon GuardDuty focuses on threat detection and anomaly monitoring across your AWS environment and it does not perform content discovery or classification of S3 objects.

Full AWS Practitioner Certification Question