Which of the following variables corresponds to a KPI for discovery campaigns?

Reach is the correct answer because discovery campaigns aim to introduce a brand or product to a broad audience that may not yet be familiar with it. Therefore, measuring the unique number of people exposed to the campaign (reach) is a primary indicator of its success in expanding brand awareness. Clicks and CTR (Click-Through Rate) are more relevant KPIs for consideration or conversion campaigns, where the goal is to drive user engagement or direct action. While clicks can occur in discovery campaigns, they are not the primary measure of success for the initial awareness objective. Traffic lift is a broader metric that can be influenced by many factors and is typically used to assess the overall impact on website visits, not solely the effectiveness of a discovery campaign in reaching new audiences.