Which environment effectively isolates untested code changes to prevent them from affecting the production repository?

The correct choice is the sandbox (development environment) because this environment is specifically designed to isolate new or untested code changes from the main production environment. In a sandbox, developers can experiment, test new features, or try out code modifications without any risk of impacting the operational systems or the stability of the production repository. This separation is crucial for maintaining system integrity and allows developers to identify issues in a safe space.

In contrast, the testing environment is primarily used for evaluating code after it has been developed but may still not offer the same level of isolation as a sandbox. The staging environment serves as a final step before production, where code is prepared for deployment but is not intended for initial development or testing of unverified code. Finally, the production environment is where the live version of the application runs, making it critical that untested changes never enter this space to avoid disruptions or outages.