The best practice for securing secrets required by applications is to use a dedicated secrets management service. This service is purpose-built for securely handling credentials, API keys, and other secret information, with support for automatic rotation and secure retrieval by authorized compute services, such as serverless functions. Environment variables can be accessed by unauthorized personnel if they gain access to the deployment environment, and they typically hold configuration data rather than sensitive secrets. An IAM role is an AWS resource that allows you to define permissions for service or user actions but does not offer a way to store sensitive credentials. Instance Metadata Service is specific to EC2 and provides temporary, short-lived metadata to EC2 instances and is not appropriate for long-term secrets management.