The correct answer is D, 'Create cost allocation tags.' Cost allocation tags are key-value pairs that can be assigned to AWS resources, allowing users to categorize and track costs based on specific attributes or dimensions, such as application, environment, or department. By assigning these tags to resources, the company can monitor the costs associated with each application within a single AWS account. This method provides visibility and insights into the costs incurred by each application, which is the requirement stated in the question. The other options, such as setting up invoiced billing (A), using AWS Artifact (B), and setting budgets in Cost Explorer (C), do not directly address the monitoring of costs by application within a single account.