• The backup image
• Nothing will serve
• A random ad
• The default ad

The correct answer is: The default ad

Explanation: Default ads serve when no other ads can serve. Here is a reason this can happen. Your standard ads are not scheduled to serve. Check the ad start and end dates for your placement. Make sure that these dates are correct. If the ad start date is in the future, the ad cannot serve yet. If the ad end date has already passed and you set a hard cut-off, the ad cannot serve anymore.
Read more here: https://support.google.com/campaignmanager/answer/2828553