Which website is recognized as a note-taking application compatible with multiple operating systems?

The website recognized as a note-taking application compatible with multiple operating systems is keepnote.org. KeepNote is designed specifically for taking and organizing notes, providing a user-friendly interface that allows users to easily create, manage, and retrieve their notes across different platforms. This versatility makes it suitable for users who work in diverse environments, as it supports various operating systems and enhances accessibility.

The other options either do not pertain to note-taking applications or serve completely different purposes. For instance, arin.net is associated with the American Registry for Internet Numbers and focuses on IP address allocations, while toolbar.netcraft.com provides web tools and services focused on web technologies. Notekeepers.net, while it sounds relevant, is not widely recognized or established as a note-taking application like KeepNote.