The correct answer is D because cost allocation tags are a way to track the infrastructure costs for each of the projects separately. Cost allocation tags are key-value pairs that can be attached to AWS resources, such as EC2 instances, and used to categorize and group them for billing purposes. The other options are incorrect because they do not meet the requirements of the question. Use a different EC2 instance type for each project does not help to track the costs for each project, and may impact the performance and compatibility of the applications. Publish project-specific custom Amazon CloudWatch metrics for each application does not help to track the costs for each project, and may incur additional charges for using CloudWatch. Deploy EC2 instances for each project in a separate AWS account does help to track the costs for each project, but it impacts the existing infrastructure and incurs additional charges for using multiple accounts. Reference: Using Cost Allocation Tags