Your colleague has customized a report and wants it to be easily viewable. Where should they add the report so it appears in the left-hand navigation?

Add it to Collection is correct because in Google Analytics 4, adding a customized report to a Collection in the Reports library makes it visible in the left-hand navigation for users of the property. Collections organize reports in a way that they are easily accessible and structured in the main menu, whereas Admin, Explore, or Library are used for configuration, data exploration, or managing the overall report library rather than directly controlling left-hand navigation visibility.