You create golden Compute Engine images but need the app to select the correct database per environment (test/staging/prod). How should you supply the DB connection info at instance boot?

The correct answer leverages Compute Engine metadata, a standard and secure way to provide instance-specific configuration. The application queries the instance's metadata server to retrieve the DATABASE value, which contains the connection string. This approach keeps sensitive information out of the image and allows for dynamic configuration per instance. Embedding the connection string in the image (first option) is inflexible and requires maintaining multiple images, which is inefficient. Using instance tags (second option) is not ideal for storing connection strings; tags are primarily for organizing and filtering resources, and querying the Compute Engine API from the application for this purpose adds unnecessary complexity and potential permission issues. While setting an environment variable (third option) is a common pattern, the prompt specifically mentions creating a metadata item, and the most direct way for an application to access metadata is by querying the metadata server, not relying on it being automatically exposed as an environment variable without explicit configuration.