The correct answer is A. Using one AWS account for each department allows for the most precise tracking of costs, as it isolates each department's usage and expenses. While cost allocation tags (option B) can provide a way to categorize and track resources, they may not capture all usage, especially for less frequently used services. AWS Trusted Advisor (option C) offers recommendations to help improve system performance and reduce costs but does not directly allocate costs by department. Savings Plans (option D) can provide cost savings over on-demand pricing, but they do not address the departmental cost allocation issue. Therefore, for exact AWS Cloud usage cost allocation by department, separate AWS accounts are the most accurate method.