Moved here as this isn't about Linux Mint.
C:, D:, etc. are how Windows names partitions on your hard disk(s). On Linux, partitions are named in a different way. /dev/sda1 is the first partition (the '1' at the end) of your first hard disk (the 'a' before that '1' at the end). The second partition on your first hard disk is thus /dev/sda2 and the first partition on your second hard disk (if you have one) is /dev/sdb1.
That cleared up, how you install Kali Linux depends on what Kali Linux's installer offers for options to help you. Linux Mint's installer for example offers you the option to install alongside existing operating systems, and it will resize the partition of those to make room for itself. Perhaps Kali Linux has something similar. Else you will have to boot from a Linux live image that has GParted or similar partitioning tool, so you can yourself first shrink your existing Linux Mint partitions to make room for the new distro to install.
Take note also of the boot loader. Currently, if you have no other OS installed, Linux Mint's boot loader will be installed onto the so-called master-boot record (MBR) of your first hard disk. This is what is loaded by your computer's BIOS. If you install a new OS, make sure you are clear about whether it will install another boot loader and if so where. If you let Kali Linux's installer do the default, it will probably install a new boot loader into the MBR--meaning if at some point in the future you want to remove or overwrite Kali Linux you will have an unbootable computer because the boot loader in the MBR no longer can find Kali Linux.
I've never heard of Kali Linux, but I'd hope they have some documentation on this. Else mayhap they have a forum?