To test a manual bidding strategy against an omnichannel Smart Bidding campaign, use Campaign Experiments.

• Geo-X
• Incrementality
• User-based
• Campaign Experiments

The correct answer is: Campaign Experiments

Explanation: Campaign experiments offer a versatile tool for testing various features in your advertising strategy. Specifically, they enable you to evaluate the efficacy of omnichannel bidding. By using this tool, you can juxtapose a manual or a non-conversion-based bidding strategy with an omnichannel Smart Bidding campaign. Based on the outcomes, you can either integrate the draft changes into your existing campaign or kickstart a completely new one.