Hi Jay, Here is how I would do with what you have and also coupled with my experience of dual-booting with Windows 10 Insider Pro Preview edition.
1. Backup anything on D: - would be useful to know how much is on there as not clear if anything is on there - GParted would give a better graphical result (will post image here - on a different machine running Windows 7 Pro as I write this reply)
[Update:
This is the layout of Windows 10 with Makulu Linux]
2. Delete D: partition completely
3. Increase your C: drive - but run chkdsk and defrag before resizing - refer to my tutorial video on vimeo. Take your C:\ drive up to 80 Gb.
4. What is left as unallocated space, create a LOGICAL partition, labelled D: and call it DATA - format to NTFS - Zorin can read NTFS partitions natively PROVIDED Windows was shut down cleanly - e.g., no forced shutdowns. This will be roughly 20 Gb so should be ample for your Windows data - then again I don't know what your intentions are - video can eat up a lot of space in no time!
5. Where D:/ is now create an EXTENDED partition - this will be where you have all of your partitions for Zorin. First off create a '/boot' partition of 512 Mb - this is where we are going to place GRUB!
5.1 Inside the Extended partition create a '/' root partition of 30 Gb - no technically correct as posted by another forum member 1 Gig =1024 so 30 would be 30 x 1024 to get the correct size and format to ext4.
6. At the END of the EXTENDED partition, create a Linux swap-area of 4 Gb (4096). You don't have to specify any file system as this is done automatically. (I think Wolfman once posted that he thought it was on the lines of FAT32).
7. What is left in the middle, format as your '/home' partition - this is where you have all your personal data stored and if you use Wine for Windows apps, this is also where the fake C:\ drive and all applications are stored.
8. Make sure that you place GRUB on the '/boot' Partition -based on my Windows 10 experience.
IMPORTANT - make a note of what sda/sdb partition number this is - but the giveaway will be the size of the partition as you won't have any other partition that size so readily recognisable in EasyBCD. Again refer to my tutorial video on how to do this - this will ensure that your Windows Boot file on mbr never gets corrupted so if anything goes wrong with Zorin you still have access to your Windows.
Fingers crossed! Keep us posted!