Answer:
The correct answer is B, Mac.
Step-by-step explanation:
OS stands for Operating System and GUI stands for Graphical User Interface. The first Operating System was System 1.0 which was the first graphical user interface operating system developed for Macintosh. This first operating system had so much features similar to ones present in the current operating systems. There were windows in that OS, files and folders could be moved with mouse, files and folders could be copied and dragged(pasted) on other locations. So first ever GUI OS was MAC.