Which of the following operating systems is open-source software? A) Macintosh Operating System B) Unix C) Win?

Linux. Durr.

B for Unix, but it really depends on the distro, not all of them are open

D for Linux always

Windows and Mac are closed source

Just Linux on this list.
There are many others too.

Linux uses a lot of commands very similar to Unix, were they copied?

D linux is Open Source Operating System

Of course Linux is open source. All of the answerers
seem to have gotten that right.

However, all but one seem to think that *only* Linux is
open source, but they are wrong.

The FreeBSD project has been around for a long time
(at least a decade, maybe two).

OpenSolaris has been open source for a few years.

See the references.

PS. While the full OS on the Mac isn’t open source,
you might note that under the covers, the kernel is
BSD Unix.