My guess would be PMMM and Worm, because they are both versions of the earth we are hopefully all somewhat familiar with.
This. Whatever's going on in those other 'contexts', we can't say for sure that reality, physics, the known universe all works the same way ours does, with the same general rules. Like Emma was saying about stars being balls of hydrogen fusion, the other contexts don't have that for sure. It's not just that they're ignorant, it's that even we as viewers of the setting can't actually say that their universe is anything like we're familiar with.
Except PMMM and worm. They both have modern cities, and cars. They use electricity to power lightbulbs, and have billions of humans spread across a planet called earth divided into a bunch of separate nations, and all that other stuff. They're both based fundamentally on reality, with some minor differences (magical girls/superpowers, both coming from an outside influence (aliens) to explain how/why their world differs from the one we live in).
It's not about which worlds could theoretically exist in the same known universe or whatever. 'Rwby is not on earth, so earth could still exist a million lightyears away making it the same context!' except... no. There's a planet with trees and grass and humans, large distinct continents separated by vast oceans. Sure, Remnant isn't Earth... except it totally is, just in a different context. That's pretty much the definition of differing contexts. It's not the multiverse in the worm sense of just being a step different, it's radically different while filling the same space/function in their different reality. It's the same place in a different context. That's what the moderator said when introducing the idea. 'Some of you may have an idea about the multiverse, this is one step beyond that.'
I can't really speak too much to the larger 'Undertale' world, and I know nothing at all about Yuri's setting, but it's easy enough to imagine the same description applies to them. They aren't actually 'Earth' in the way that we'd recognize it, but they're definitely Earth-like in the only real way that matters. It's the place where the humans live. It's where they evolved (or were created?), it's their home planet, by whatever name. It's Earth in a different context.
That's true of all of them, except for the two that are explicitly Earth. Those are earth in the same context.