The correct answer is 'AWS Cost Explorer' because it provides an interface for visualizing, understanding, and managing your cloud costs and usage over time. Users can dive into detailed reports that help them analyze expenditures by service, usage type, or other dimensions. This examination of past spending is critical for formulating cost-optimization strategies. Other options like 'Budgets' are primarily for setting thresholds for expected future spending rather than for analyzing past spending details. 'Pricing Calculator' is designed for cost estimation before deploying services, and 'Documentation' refers to a wide range of guides and technical manuals rather than a focused tool for financial management.