AWS Cost Explorer is the correct answer as it provides a detailed view of the company's AWS costs and usage over time, including the ability to visualize spending on specific services like On-Demand and Spot Instances. This tool allows users to analyze their spending patterns, which is essential for understanding and managing costs associated with different types of EC2 instances. The other options, such as AWS Budgets, Amazon CloudWatch, and AWS Cost Categories, do not offer the same level of detailed cost visualization and analysis for EC2 instances as AWS Cost Explorer does.