- Default ad
- .swf file
- Backup image
- Polite load image
The correct answer is: Polite load image
Explanation: Upload a polite load image so that you have something to show if there are delays loading your primary HTML5 asset to a browser. When your creative is delivered, the polite load asset loads first before each impression, followed by your primary HTML5 asset. This way, if the user’s browser experiences a delay loading your primary asset, the polite load image will be there until everything else finishes loading.
Read more here: https://support.google.com/campaignmanager/answer/4558694