It is unknown what piston will come in the piston kit for your 2007 YZ125. If you specifically want the D piston you must piece together the kit from the OEM microfiche. Unless you are running a new cylinder though, you can utilize any of the 4 pistons in your current cylinder and they will all work just fine.
The second to last digit in the part number will specify which piston you are ordering. They will all use the same piston ring.