_____ represent(s) the percentage of people who are exposed to an online ad that actually clicked their mouse on it.

The correct answer is click-through rates (CTR), which represents the percentage of people who, after being exposed to an online advertisement, actually clicked on it. Click-through rates are crucial for evaluating the effectiveness of online advertising campaigns. They provide insight into how well the advertisement captures the attention of the target audience and encourages engagement.

A higher CTR indicates that the ad resonates more with viewers, leading to increased traffic to the advertiser's website or landing page. This metric helps marketers assess the performance of their ads and make informed decisions about future campaigns, such as adjusting targeting strategies, changing ad creative, or reallocating budgets.

In contrast, opportunity to see (OTS) refers to the total number of times an advertisement could be seen by potential viewers, not accounting for actual engagement. Cost per action (CPA) relates to the cost incurred by the advertiser only when a specific action is taken, such as a purchase or lead generation, rather than merely a click. Search engine assessment (SEA) is not a standard term in advertising metrics and doesn’t define the relationship between ad exposure and engagement.