To access Windows drive or folder in your Guest OS, you first of all, install VirtualBox guest utilities:
sudo apt-get install virtualbox-guest-utils
You then Add a shared folder by going into: Settings > Shared Folders > Click the Plus Icon, and add the folder location. Having done that, you should check Auto Mount and Make Permanent.
Lastly, add your user to vboxsf group:
sudo usermod -aG vboxsf $(whoami)
Restart Virtual Box, and you should be good to go.
In Ubuntu, you can check the mounted drive by entering "df -h"