What Debian version are the different Ubuntu versions based on?
You can find the Debian version on which your Ubuntu version is based in the file: /etc/debian_version
From 10.04 up to 21.04:
Ubuntu Debian
21.10 impish bullseye/ sid - 11
21.04 hirsute bullseye/ sid
20.10 groovy bullseye/ sid
20.04 focal bullseye/ sid
19.10 eoan buster / sid - 10
19.04 disco buster / sid
18.10 cosmic buster / sid
18.04 bionic buster / sid
17.10 artful stretch / sid - 9
17.04 zesty stretch / sid
16.10 yakkety stretch / sid
16.04 xenial stretch / sid
15.10 wily jessie / sid - 8
15.04 vivid jessie / sid
14.10 utopic jessie / sid
14.04 trusty jessie / sid
13.10 saucy wheezy / sid - 7
13.04 raring wheezy / sid
12.10 quantal wheezy / sid
12.04 precise wheezy / sid
11.10 oneiric wheezy / sid
11.04 natty squeeze / sid - 6
10.10 maverick squeeze / sid
10.04 lucid squeeze / sid
Sid is the development distribution of Debian.
You can find out the contents of the file without installing an entire system by view the sources for the package basefiles on Launchpad.
Unfortunately there is no direct mapping /etc/debian_version as mentioned in Jacob's answer gives a rough idea but it only really reflects one package and said package is a package that is locally modified in Ubuntu so won't be automatically pulled in from Debian.
In particular Ubuntu is often ahead of Debian on core packages like libc6. Trying to install a package built on Ubuntu on a contemporary version of Debian is likely to end up with version errors on libc6.
According to https://superuser.com/questions/407745/wrong-libc6-version-in-debian-squeeze-can-i-go-back Debian squeeze had libc6 2.11 . According to http://web.archive.org/web/20110705212424/http://packages.ubuntu.com/search?keywords=libc6 ubuntu 10.04 lucid also had libc6 2.11 . So the packages for 10.4 lucid are IMO the most likely to work on Squeeze.
Depending on what other libraries your program needs you may well find that a specific build for Debian squeeze is needed.