The correct answer is A. Open source.
Linux is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged in a Linux distribution.
Linux is a Unix-like operating system. Unix is a multitasking, multiuser computer operating system that exists in many variants. The original Unix was developed at AT&T’s Bell Labs research center by Ken Thompson, Dennis Ritchie, and others.
Microsoft is a multinational technology company that produces computer software, consumer electronics, personal computers, and related services. Its best known software products are the Microsoft Windows line of operating systems, the Microsoft Office suite, and the Internet Explorer and Edge web browsers. Its flagship hardware products are the Xbox video game consoles and the Microsoft Surface lineup of touchscreen personal computers. As of 2016, it is the world’s largest software maker by revenue.
Windows is a group of several proprietary graphical operating system families, all of which are developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. Active Windows families include Windows NT and Windows IoT; these may encompass subfamilies, e.g. Windows Server or Windows Embedded Compact (Windows CE). Defunct Windows families include Windows 9x, Windows Mobile and Windows Phone.
Mac is a line of personal computers designed, developed, and marketed by Apple Inc. since 1984. The original Macintosh was the first mass-produced personal computer to feature a graphical user interface, which allowed users to interact with the computer using a mouse instead of typing commands. The Macintosh was a commercial success, and Apple went on to release a number of other models, including the iMac, MacBook, and Mac Pro.
In conclusion, the correct answer is A. Open source.