What is the best description of the ad which is illustrated in the following image?

It is a sample ad unit used during development. It has fixed sample ad unit ID :ca-app-pub- 3940256099942544/6300978111

It will appear in the app activity without the need to configure neither the ad unit nor the app Id.

It is an option that can be selected through AdMob web site, then this test sample ad unit id will be sent to your email; therefore, it is not fixed.

This is not an ad, it is image already available with Android Studio images. Developers add it to reserve a space for the real ads which will be configured later.