The correct answer is D, AWS Cost Anomaly Detection. This service uses machine learning to continuously monitor your AWS cost and usage, identifying unusual spending patterns. It learns your unique spending patterns and automatically detects any deviations from the expected behavior, helping you to identify cost optimization opportunities and prevent unexpected cost overruns. The other options are not designed for this purpose: A (Amazon Lookout for Metrics) is for detecting anomalies in metrics, B (AWS Budgets) is for setting custom budgets to manage costs, and C (Amazon CloudWatch) is for monitoring resources and applications.