The correct approach is D because deploying a new version without making it the default prevents any service interruption. Traffic splitting enables the redirection of a small portion of traffic to the new version, allowing for real-world testing without significant risk. This method also facilitates quick reversion if issues arise, ensuring minimal impact on users.

• Option A is incorrect as deploying the new version as the default would shift all traffic to it, potentially affecting all users and risking service downtime.
• Option B is not ideal because setting up a separate project necessitates additional steps like data synchronization and external traffic management, which are unnecessary with App Engine's built-in capabilities.
• Option C is unsuitable because App Engine services are designed for distinct functionalities. Using separate services would require manual adjustments from the clients' side to manage access, complicating the deployment process.