BSD
A) Free BSD is the operating system BSD(Berkeley's Software Distribution) Linux and Ubuntu are just like kernel.
Linux and BSD are both excellent and available free of charge.
There are Windows Server, Linux, BSD, and even Apple's own server
Linux is a Unix-like system. This means that it is inspired or influenced by Unix in some shape or form (Linux started off from Minix), but it is not directly derived from Unix. However, BSD is based on Unix, and macOS is indirectly Unix-based because of its mixed heritage with BSD.
Yes, though Linux does a pretty good job of running most of the Internet, the Internet has been around a lot longer than Linux has. IF it wouldn't be Linux, it'd probably be BSD.
PC-BSD is a distribution of FreeBSD, a free and open-source Unix-like operating system. PC-BSD is focused on desktop use, and features the KDE desktop environment, a GUI popular with Linux distributions. Probably the most distinguishing feature of PC-BSD is the method of installing software. FreeBSD and most Linux distributuions use a package manager to download and install software. PC-BSD software is installed in a more Windows-like manner. Installers, with the extension ".pbi", are downloaded from the PC-BSD website. The user just double-clicks them to run the installer and install the program.
OSX BSD Linux Microsoft Windows DOS OS/2
Linux, Minix, Coherent, FreeBSD, etc. These are all clones of Unix
GNU/Linux, and the BSD descendants follow design and operation principles largely similar to UNIX.
This obviously depends what platform your writing your program for, if it's for Linux/BSD/Solaris etc, use Linux. If it's for Windows guess what you use? Windows (Except in some cases using Linux is okay). If it's for... you get the point. Personally, I use Linux.
Linux, and the BSD-derived versions (FreeBsd, PCBsd, etc.)
It's about as easy to use as any Linux distribution. The main point it lacks in is hardware support, which isn't quite as good as Linux's. It uses the KDE desktop, which is also found in many Linux distributions.