Option B is correct because it allows for traffic splitting within the same App Engine application, enabling you to test the new UI with a subset of users before a full rollout. This method is efficient and does not require migrating to a different platform or managing separate applications. Options A and C suggest unnecessary migrations to Google Kubernetes Engine and Compute Engine, respectively, which are not required for this scenario. Option D incorrectly suggests deploying a new version as a separate app, which complicates the process without providing any benefits over the correct approach. For more details, refer to App Engine traffic splitting documentation.