The correct answer is C because deploying a new version without assigning it as the default version avoids downtime for the application. Traffic splitting allows for directing a small percentage of traffic to the new version, enabling testing in a live environment with minimal risk. This approach also facilitates quick reversion without impacting the entire user base.

• Option A is incorrect because deploying the application version as default would shift all traffic to the new version, potentially affecting all users and causing service disruption.
• Option B is not advisable as it could lead to service disruption and is not a scalable solution for managing updates.
• Option D is not recommended as it negatively impacts user experience and does not provide a technical solution to mitigate risk.

For more details, refer to the GCP documentation on App Engine Splitting Traffic.