Which is the ground wire in my lighting circuit?

Odds are excellent that you have an additional bare wire and cable sheath out of sight up the ceiling. And that your "yellow" wire is actually "White, but old and yellowed."

This is a crappy install, in that the cable sheaths should have been coming all the way into the "old fixture as junction box" and if they don't make it to the hole, that obviously wasn't happening, and the next thing in line wasn't grounded, (and I doubt the first thing was given how short that ground wire is cut,) unless that cable has just retreated into the hole after you removed the fixture. Bare copper or green are grounds.

It's marginally possible but highly unlikely that the ballast in one fixture was running a tube in a separate fixture (where yellow would be a reasonable likelihood as a real color.) If so, you're going to need to replace that wire anyway.

For cables used in North America, the conductors are black, white, bare, then black, white, red, bare, etc - i.e. you won't find a yellow wire in a normal household cable. You might in a conduit. But the yellowed/aged white wire is a much more likely case.