The correct answer is A because the requirement is to encrypt data in transit and data at rest while ensuring that the developer can manage encryption keys . AWS Key Management Service (AWS KMS) is the AWS-managed service specifically designed for creating, controlling, and auditing cryptographic keys. When used with the AWS Encryption SDK , it supports envelope encryption , a recommended pattern for protecting sensitive application data.

In this model, the application uses data keys to encrypt the actual sensitive data, while AWS KMS protects and manages the master keys that encrypt those data keys. This provides strong security and centralized key management, including access control through IAM, key rotation options, and auditability through AWS logging integrations. For data in transit , the application should also use TLS/HTTPS , which is standard AWS guidance for secure communications.

Option B is incorrect because Parameter Store is not the correct mechanism for directly managing application encryption keys in this way. SecureString protects stored values, but it is not a substitute for using KMS as the primary managed key service for encryption architecture. Option C is incorrect because Secrets Manager is intended for storing secrets such as passwords and tokens, not as the main key management system for application encryption design. Option D is incorrect because SSE-S3 uses Amazon S3-managed keys, not customer-managed KMS keys, so it does not satisfy the requirement that the developer be able to manage encryption keys.

Therefore, AWS KMS with envelope encryption through the AWS Encryption SDK is the most appropriate and secure solution.