Explanation:

An operating system (OS) is software that manages computer hardware and software resources and provides common services for computer programs. Unix is a family of multitasking, multi-user computer operating systems that originated in the 1960s, making it a clear example of an OS. Dot Net (.NET) is a software framework developed by Microsoft, used for building various applications, but it is not an OS itself. Java is a high-level, class-based, object-oriented programming language, used to develop software, not an operating system. Maya is a 3D computer graphics application used for animation, modeling, simulation, and rendering, falling under application software. Thus, only Unix is an operating system.