An advertiser is launching a Conversions campaign to increase checkouts during the holiday season, but is unwilling to bid more than $23. What bidding solution must the advertiser use?

Custom bidding is the correct solution because it allows advertisers to set a maximum bid for specific actions, such as checkouts, which directly addresses the advertiser's requirement of not wanting to bid more than $23. This gives the advertiser precise control over their spending per conversion. Dynamic retargeting focuses on re-engaging users who have previously interacted with the advertiser's content, but it doesn't directly control the bid amount per conversion in the way custom bidding does. Performance+ bidding and Performance+ targeting are broader Pinterest ad solutions that optimize for performance but do not offer the granular control over a maximum bid for a specific conversion event that custom bidding provides.