Kevin provides a good explanation of the different options. For your scenario I would make the following suggestion:
This isn't always required for installing Ubuntu but because you have a busy setup going on, we wanna fine tune our partitions before starting the setup. Just so we can be sure of what's gonna happen with our data and drives and partitions etc.
Boot your Live USB/CD into "Try Ubuntu" mode:
- Open a program called GParted
- Select your G: Drive in the top right corner, in this case it will be named according to Linux methodology. Something like /dev/sde
- Resize until you have the 15GB free space Unallocated
- Create a 2GB partition as Swap. Learn more here
- Create a partition in the remaining space as ext4
- Remember to Apply the changes!
During the Installation process (called Ubiquity), at the partitioning step:
- Select Something Else and click next.
- Then double-click the prepared ext4 partition on your intended drive for mounting "/". We call this the root structure, the entire file system falls under this. Just like a C: drive in Windows. You can tick the format box if you want even though we just did it.
- The swap partition should automatically be detected, you can double click to check if it's "used as swap area".
- The bootloader can also go on the 'G: drive' so we don't have to bugger up the windows bootloaders. Underneath the partitions, in the dropdown, select /dev/sde or whatever this one may be.
- Continue installation.
- IMPORTANT: When restarting, open your BIOS and select your G: drive as the primary boot drive so that it loads the linux bootloader (Called GRUB) before the Windows ones. Grub allows you to select which operating system you want to boot (Dual/Triple boot) and will use Ubuntu as default, you can always change this using grub-customizer
Now we should have installed Ubuntu alongside the rest without messing with the other drives or their operating systems.
Let us know if this worked for you! Good Luck!