The correct answer is B, AWS Budgets. AWS Budgets is a service that allows customers to set custom budgets for their AWS usage costs and usage amount, and receive alerts when actual costs exceed those budgets. It provides the functionality to set alerts based on cost thresholds, which is the requirement in the question. AWS Cost Explorer (A) is used for visualizing and understanding AWS costs, but it does not provide event-driven alerts. Cost allocation tags (C) are used for identifying and organizing AWS costs, but they do not provide cost tracking or alerting capabilities. AWS Cost and Usage Reports (D) provide detailed cost and usage information, but they do not offer the alerting functionality needed to notify when costs exceed specific thresholds.