An advertiser should create a default ad in Campaign Manager 360 to ensure a creative is always available to run, even when no other ads are eligible.

• They can set up their geographic target for a specific region.
• They should create a default ad so that it is always available to run.
• They can produce an ad with a region-specific creative.
• They can enable their account to serve across mobile devices only.

The correct answer is: They should create a default ad so that it is always available to run.

Explanation: Default ads serve as a backup when no other ads meet the eligibility criteria for a placement. They have no targeting restrictions, allowing them to be shown to any user. This ensures continuous ad delivery, preventing gaps caused by targeting mismatches, scheduling limitations, or impression caps. By setting up a default ad, advertisers can maintain consistent engagement and maximize their campaign’s effectiveness. Default ads are automatically assigned when a compatible creative asset is added, ensuring that an ad is always displayed. (support.google.com)