What is necessary for creating a Custom Audience from website visitors?

To create a Custom Audience from website visitors, the essential requirement is the Facebook pixel. The Facebook pixel is a piece of code that you place on your website, which allows you to track user behavior, such as page views, conversions, and other specific actions that visitors take on your site. By collecting this data, the pixel enables marketers to build highly targeted Custom Audiences based on actual interactions and behaviors of website visitors.

The other options mentioned do not serve the necessary function. For instance, an Instagram login is relevant for managing Instagram advertising but is not required for tracking website visitors or creating Custom Audiences. An Ad Account ID is related to the management of ads and campaigns but does not directly influence the ability to create a Custom Audience from website visits. Lastly, user engagement data can be important for various marketing strategies, but it is not a standalone requirement for generating a Custom Audience without the support of the Facebook pixel's tracking capabilities. Thus, the Facebook pixel serves as the foundational tool needed to effectively build a Custom Audience that reflects the behaviors of users on a website.