Define a 'viewable impression'.

A 'viewable impression' is defined as an advertisement that becomes viewable when a minimum of 50% of its pixels are displayed on the user's screen for a duration of at least one second. This definition is crucial within the digital advertising industry as it sets a standard for measurement that helps advertisers assess the effectiveness of their ad placements.

This standard was established by the Interactive Advertising Bureau (IAB) and the Media Rating Council (MRC), which aim to ensure that advertisers can be confident that their ads are being properly viewed by users. The importance of this definition lies in its reflection of actual user exposure, as ads that are not visible are unlikely to generate engagement or impact brand awareness.

In contrast, the other options do not accurately capture the definition of a viewable impression as recognized in the industry. For instance, merely generating high user engagement or leading to a click-through does not constitute a viewable impression if the ad was not displayed adequately. Similarly, simply being visible for one second without the requisite percentage of visibility does not fulfill the criteria for a viewable impression. Therefore, the correct answer encapsulates the precise requirements set forth for an ad to be considered viewable.