The correct answer is B. AWS allows you to apply tags to your resources to help organize them and identify their purpose. By using tags to categorize resources by department and application, you can easily track AWS costs on a detailed level. AWS also offers cost allocation tags, which enable you to categorize your costs by department or application. By activating cost allocation tags, you can view and analyze your AWS costs in greater detail. While options A, C, and D all offer ways to manage and track AWS costs, they do not specifically address the need to categorize costs by department and application as required by the company in the question.