Which of the following operating systems is based on UNIX source code and is closed source?

Android is based on the Linux kernel, which is UNIX-like in design but was written independently without using original UNIX source code, and it is distributed as open source under the Apache License.

Windows is a closed-source OS but is not based on UNIX source code - it is derived from the Windows NT kernel architecture, which has no UNIX lineage.

macOS is built on Darwin, which incorporates BSD (Berkeley Software Distribution) code - a direct descendant of the original AT&T UNIX source code. Apple distributes macOS as a proprietary, closed-source product, and it holds official UNIX certification from The Open Group, satisfying both criteria of being UNIX source-based and closed source.

Linux was written entirely from scratch by Linus Torvalds without using UNIX source code, and it is distributed as open source under the GNU GPL license.