To ensure an appropriate ad runs every time in Campaign Manager 360, create a default ad that can be shown to any audience when no other ads are eligible.

• Create an ad with multiple region-specific creatives.
• Create a default ad so that any audience can view it if it needs to be shown.
• Set your geographic targeting to only a small amount of regions across the United States.
• Set your account to only serve across mobile devices.

The correct answer is: Create a default ad so that any audience can view it if it needs to be shown.

Explanation: Default ads serve as a backup in Campaign Manager 360, ensuring that an ad is always displayed when no other ads are eligible. These ads do not have targeting restrictions, making them available to all users. Setting up a default ad prevents empty placements, ensuring continuous ad delivery even if other ads fail due to scheduling gaps or targeting mismatches. This helps maintain engagement and optimizes ad exposure. Default ads are automatically assigned when compatible creative assets are added to a placement. (support.google.com)