Which operating system (OS) is a clone of Unix?

Linux is considered a clone of Unix because it was designed to be a Unix-like operating system, mimicking the functionality and interface of traditional Unix systems. Linus Torvalds created Linux in the early 1990s as a free and open-source alternative to Unix. It incorporates many of the same principles, commands, and programming interfaces that characterize Unix, allowing users and developers familiar with Unix to adapt to Linux seamlessly.

This association with Unix gives Linux its robust multi-user support, multitasking capabilities, and overall reliability, which are hallmark traits of Unix systems. Additionally, the availability of the Linux kernel has led to its adoption in various distributions, all existing in the Unix family tree, often considered as Unix-like operating systems.

The other options do not share this specific lineage or design philosophy. While Solaris is based on Unix and has a Unix-like structure, it is not a clone as it is specifically developed by Sun Microsystems and has its own proprietary elements. Windows is a completely different operating system that does not derive from Unix, and "Kernel" is a term that refers to the core component of an operating system but does not specifically denote an OS itself, particularly not a Unix clone.