In which scenario is incremental it y the measurement option that an advertiser should use?

The selection is correct because incrementality measurement is specifically designed to determine the "causal lift" of an advertising channel, helping an advertiser decide if shutting off paid search is appropriate by identifying which conversions were truly driven by the ad versus those that organic search would have captured anyway. Unlike standard attribution, which simply assigns credit to touchpoints, incrementality uses controlled experiments—comparing a treatment group (exposed to ads) against a control group (not exposed)—to isolate the specific volume of sales or leads that would not have occurred without the paid investment. This allows the advertiser to move beyond correlation and understand the actual value added by their paid search budget, preventing them from paying for conversions they would have received for free through organic results.