Which feature will help David manage multiple applications and online references at the same time?

The best choice for helping David manage multiple applications and online references simultaneously is Split View. This feature allows users to view and interact with two applications side by side on the screen, effectively utilizing screen space and enhancing productivity. By displaying two active windows at once, Split View makes it easier to compare information, drag and drop content between applications, or simply multitask without having to constantly switch back and forth between different application windows.

While the other options provide useful functionalities, they cater to different use cases. Tabs in Safari are great for managing multiple web pages within a single browser window but do not facilitate multi-application management. Spaces in Mission Control allows users to organize applications into different virtual desktops, which can enhance organization but does not directly enable simultaneous access. Notes is a productivity application used for capturing ideas and information but does not serve the purpose of managing multiple applications at once. Thus, Split View is clearly the most suitable feature for managing multiple applications and online references effectively.