Answer
Macintosh Operating System.
Step-by-step explanation
The Finder is the default file manager and graphical user interface shell used on all Macintosh operating systems. The finder on Mark's computer was always running because the finder is responsible for the launching of other applications, and for the overall user management of files, disks, and network volumes. Macintosh Operating System is the computer operating system for Apple Computer's Macintosh line of personal computers and workstations.