To address security concerns about sensitive information such as usernames and passwords in the application code, the best approach is to utilize environment variables. This is a common and recommended practice to enhance security because:

Environment Variables: Storing sensitive information in environment variables helps prevent exposure of these details in the codebase. They can be easily managed and changed without modifying the code.

Separation of Configuration from Code: This approach follows the principle of separating configuration from code, making the application more secure and easier to manage.

Security Best Practices: Using environment variables aligns with security best practices for managing secrets, which include not hard-coding sensitive data and keeping them outside the source code repository.

Hence, using environment variables for storing sensitive details like usernames and passwords (Answer B) is the appropriate and secure solution.