You already identified the partitions correctly. If you want it on C then choose sda2, if on D then choose sda4. If it were me, I’d go with sda4 (no special reason, just personal preference) but the choice is entirely yours.
I recommend seeing this for partitioning: Partitioning during installation
I also recommend using EasyBCD to setup your boot: EasyBCD - NeoSmart Technologies
REMEMBER: Whatever you choose must be selected in that dropdown menu in the lower half of the screen.
ADDITIONAL EDIT: Actually, if you only have 53 GB on your windows drive (sda2) you almost certainly want to install Zorin on sda4.
HOPEFULLY THE FINAL EDIT: I guess I was right in my earlier post, regarding sda,sob being the drive names. As this article confirms, sda are all one drive with different partitions. Regardless, I still recommend picking the sda4 partition.