Reverting to a Prior Version of an Application

D.

The correct answer for the given question is C. On the App Engine Versions page of the GCP Console, route 100% of the traffic to the previous version.

Explanation:

When deploying an application on App Engine, it's possible to have multiple versions of the same application running simultaneously. App Engine provides a feature called Traffic Splitting which allows you to control the amount of traffic sent to each version of the application.

In case of a faulty deployment, the quickest way to revert to a previous version is to route 100% of the traffic to the previous version using the Traffic Splitting feature. This can be done using the following steps:

1. Go to the App Engine Versions page on the Google Cloud Console.
2. Find the version that you want to roll back to and click on it.
3. Click the "Migrate Traffic" button.
4. Choose the "Split traffic" option and move the slider to route 100% of the traffic to the previous version.
5. Click "Save".

By doing this, all incoming traffic will be directed to the previous version of the application until the issue with the current version is resolved.

Option A, "Run gcloud app restore", is not a valid command and does not revert an App Engine deployment to a previous version.

Option B, "On the App Engine page of the GCP Console, select the application that needs to be reverted and click Revert", is not a valid action. The App Engine page on the GCP Console does not have a "Revert" button.

Option D, "Deploy the original version as a separate application. Then go to App Engine settings and split traffic between applications so that the original version serves 100% of the requests", is not the best solution in this scenario as it requires deploying a new application instead of simply reverting to the previous version.