Which version of Ubuntu is my Linux Mint installation based on?

You'll find Ubuntu version in the /etc/upstream-release/lsb-release file:

$ cat /etc/upstream-release/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=14.04
DISTRIB_CODENAME=trusty

To figure out which subrelease you are using, you need to know what kernel you are running, e.g. here kernel 3.19:

$ uname -r
3.19.0-32-generic

Then you compare it with the 14.04.x Ubuntu Kernel Support schedule which says that in my case, the 3.19 kernel matches 14.04.3. Now on wiki it listed

I've done /etc/os-release. Here is the result with extended information:

NAME="Linux Mint"
VERSION="18.3 (Sylvia)"
ID=linuxmint
ID_LIKE=ubuntu
PRETTY_NAME="Linux Mint 18.3"
VERSION_ID="18.3"
HOME_URL="http://www.linuxmint.com/"
SUPPORT_URL="http://forums.linuxmint.com/"
BUG_REPORT_URL="http://bugs.launchpad.net/linuxmint/"
VERSION_CODENAME=sylvia
UBUNTU_CODENAME=xenial