The correct answer is D. Encrypting data by using AWS Key Management Service (AWS KMS) is the most direct approach to enhancing a user's security on AWS. AWS KMS is a managed service that allows users to create, control, use, and manage encryption keys. By encrypting data, it ensures that even if there is unauthorized access, the data remains protected and unreadable without the correct encryption key. This method adds an extra layer of security to sensitive data, maintaining data confidentiality and integrity. While options A, B, and C are also important for overall security and architecture, they do not directly address the encryption of data, which is a fundamental aspect of security.