Storing credentials in the source code (A) is insecure because it makes them discoverable in plain text to anyone with access to the source code. Using a config file with restricted access (B) is better but still requires manual updates and poses risks if the file contains plain text credentials. Environment variables (C) are not ideal because they need to be available in plain text when the session starts. The most secure option is a secret management system (D), which handles credentials securely by generating, using, rotating, encrypting, and destroying cryptographic keys, and managing permissions to those keys.

For more information, visit:

• Google Cloud Secret Manager
• Google Cloud Secret Manager Documentation