The correct answer is "UBUNTU".UBUNTU: UBUNTU is a complete LINUX operating system.It is free to use and open-source operating system.ZENIX:ZENIX is a discontinued version of UNIX operating system.It is not a free open-source operating system.Windows 7:Windows 7 is an operating system developed by Microsoft.To use this, we have to buy its license and it is not open source. UNIX:UNIX is a family of multitasking, multiuser, computer operating system.It is not a free and open-source operating system.Additional InformationOperating system is a set of programs that links between the user and the computer.LINUX is a free and open-source operating system. Some LINUX based operating systems are UBUNTU, MINT, BOSS, FEDORA etc.