Just select the guest operating system that will run in the virtual machine.
The operating system choice is used to name the virtual machine, and to select the amount of memory with which to configure the virtual machine. Guest operating systems that are not listed in the pull-down selector may also run, but may not be officially supported by VMware.
Unsupported operating systems can be installed by choosing the Other option. More detailed technical information on supported guest operating systems is available on the VMware support web site.