Which of the following operating systems is not based on microkernel system structure? (A) Mach (B) MINIX (C) HURD (D) QNX (E) UNIX

The question is asking to identify which of the listed operating systems is not based on a microkernel architecture. Here's the analysis:

1. Mach: This is a microkernel-based operating system.
2. MINIX: Also designed as a microkernel-based operating system.
3. HURD: Built on top of the Mach microkernel, hence it follows a microkernel architecture.
4. QNX: It is another example of a microkernel operating system.
5. UNIX: Traditional UNIX systems are monolithic kernels, not microkernels.

Final Answer

Based on the analysis, the operating system that is not based on microkernel system structure is (E) UNIX.