Which of the following can be defined as a custom winning conversion in an AB test?

The choice of defining click email or visit landing pages as a custom winning conversion in an AB test is appropriate because these actions directly measure user engagement and interaction with a campaign's goals. In the context of an AB test, which is designed to compare two or more variations to determine which one performs better, clicks on emails and visits to landing pages are definitive indicators of how successfully the variations drive potential customers towards desired actions.

Tracking these conversions allows marketers to assess which versions of their campaigns are more effective in achieving the overall objective, such as generating leads or increasing traffic to a specific page. This makes them clear and quantifiable metrics for evaluation.

In comparison, while other actions such as forwarding an email to a friend, engaging with social media posts, or signing up for a newsletter can indicate some level of interest or engagement, they do not directly reflect the primary conversions that a marketer is often interested in for the evaluation of an AB tested campaign. Thus, focusing on clicks and landing page visits provides a clearer pathway to determining the success of the variations being tested.