A marketer should create a default ad in Campaign Manager 360 to ensure an ad is always displayed, even when no other ads are eligible to serve.

• They can set a geographic target for a specific range of regions.
• They can create an ad with many region-specific creatives.
• They should create a default ad so any audience can see it whenever it’s needed.
• They can set the account to serve exclusively across smartphone mobile devices.

The correct answer is: They should create a default ad so any audience can see it whenever it’s needed.

Explanation: Default ads act as a fallback when no other ads meet the eligibility criteria for a placement. Since they have no targeting restrictions, they can be shown to any user, ensuring continuous ad delivery. This prevents empty placements due to targeting mismatches, scheduling gaps, or impression caps. Setting up a default ad guarantees that an appropriate ad is always available, maintaining user engagement and maximizing campaign reach. Default ads are automatically assigned when a compatible creative asset is added. (support.google.com)