Explanation

AWS Cost and Usage Reports (also known as AWS Cost Explorer) is the correct service for viewing detailed billing activity that occurred 3 months ago. Here's why:

• AWS Cost Explorer provides access to up to 13 months of historical cost and usage data
• It allows customers to analyze their AWS spending patterns over time
• For Amazon EC2 specifically, it shows detailed billing information including instance types, regions, and usage patterns

Why the other options are incorrect:

• A. Amazon EC2 dashboard: This shows operational metrics and instance status, not detailed billing history
• C. AWS Trusted Advisor dashboard: This provides cost optimization recommendations and best practices, but not detailed historical billing data
• D. AWS CloudTrail logs: These contain API activity logs for security and compliance purposes, not billing information

As mentioned in the AWS documentation, Cost Explorer is a free tool that allows viewing cost data for up to the last 13 months, making it perfect for analyzing billing activity from 3 months ago.