Is Unix not a PC operating system? [closed]

Without a hard definition of what a "PC" is, your assignment question is a lake of ambiguity. I used an AT&T 3B1 no later than 1987, which unequivocally ran UNIX and was marketed as a "UNIX PC".

It would depend on your definition of 'PC operating system' and subsequently on your definition of a personal computer itself. If the first refers to operating systems that run only on personal computers, you might as well rule out every imaginable one, including Windows, which also runs on mobile phones, PDAs, server machines, supercomputers and a bunch of other non-PC machinery. To my knowledge, there are no operating systems meeting this strict criterium, as part of the PC's definition may be in its use. An OS will or will not support a machine regardless of that.

Even a more objective, hardware-only classification will fail. There is no unambiguous set of PC hardware. You will need a more specific term, such as 'x86-64 architecture', but those are not necessarily the same. For instance, your professor's example of AIX not being compatible with PC hardware is false. AIX supports the PowerPC platform which is uncommon, but perfectly usable in PCs.

Any answer to a question as vague as 'name three PC operating systems' can be shot down.