The C2 version is the latest edition of that processor or in CPU lingo the latest "stepping" or "revision".
The main difference between the C2 and the original is that the C2 version has a working VT-d - This is an important feature if you plan to run virtualization software such as HyperV or VMWare Workstation or VMWare ESXi on your CPU. It will allow direct access to your systems I/O which means your virtualization software doesn't need to emulate these devices (direct pass through) and this results in faster virtual machines.
It is completely safe to get the C2 version it is the best one.