Hi small-user,
I’m a CentOS user, so a apologize if what I say is incorrect. The Jesse folder and installation is meant for a Debian 8 install, the Strech folder contains installs for Debian 9. I’m unsure about unstable and testing, but I assume it’s for development builds.
I’m having some issues seeing if Lubuntu is debian 8 or 9, but i’d say try one, starting with the Strech one, see if that one works? If it doesn’t you’ll find out soon enough and can try the other one.