Re: partition plan
It's never "safe" to remove ANY partitions that are already present on a preinstalled Windows PC -- without first confirming two things:
1) What is IN the partition
2) You don't need the stuff in that partition
My guess, without seeing the PC, is that "D" is a Recovery partition, containing a compressed Windows image file, that could be used to restore your PC to its original state (know as a factory reset).
Win8 has some kind of "refresh" option in its new boot manager. It probably uses the contents of this partition for that to work. If you remove this partition, that's probably NOT going to work.
Ubuntu 16.04 Mate, Mint 18 Mate; MS Win 8.1, MS Win10 Pro.
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