The correct answer is C. Using AWS Budgets is the most cost-effective and straightforward way to meet this requirement. AWS Budgets allows you to create a cost budget for each account with a specific scope such as EC2 instances, and it can send notifications through Amazon Simple Notification Service (SNS) when the defined spending threshold is exceeded. This approach provides timely alerts and does not require complex setups like integrating multiple services or creating detailed reports, ensuring both effectiveness and cost-efficiency.