What feature is essential for enabling users to search for layers across different projects in ArcGIS Online?

The feature that is essential for enabling users to search for layers across different projects in ArcGIS Online is the Search For Layers drop-down menu. This functionality allows users to easily find and incorporate layers from a wide array of sources beyond just their current project. By utilizing the menu, users can efficiently browse through available layers, including those shared publicly or within their organization, thereby enhancing the workflow and making it easier to add relevant data to their maps and projects.

The other options, while useful for managing or finding content, do not specifically serve the purpose of searching for layers across various projects. The Portal tab in the Project Pane is primarily for managing portal connections, the My Content section is focused on personal or shared content rather than a broader search, and the Living Atlas connection offers access to curated content but does not function as a general search tool across multiple projects.