Explanation:

The question asks to identify which of the listed options is not a system software. System software is a type of computer program that is designed to run a computer's hardware and application programs. It includes operating systems, device drivers, and utilities. Let's evaluate each option: (a) UNIX is a family of multitasking, multi-user operating systems, which is a prime example of system software. (b) LINUX is an open-source operating system kernel and a full operating system, also categorized as system software. (c) WINDOWS 10 is a major release of the Microsoft Windows operating system, making it system software. (d) AUTOCAD is a commercial computer-aided design (CAD) and drafting software application. It is used by architects, engineers, and construction professionals to create precise 2D and 3D drawings. AutoCAD is an application software, designed for specific user tasks, rather than managing the computer's fundamental operations. Therefore, AutoCAD is not a system software, making it the correct answer.