2 is pretty deep actually. And for me, that's the one I favor (though I don't dismiss 3).

Humans are fundamentally different because that's us. And even between humans, there is a scale. To me, the only real pain is mine. But, I also have the ability to feel other people's pain, so the pain of people close to me is also real, but less so, it is indirect. To feel pain for a random human requires some active thinking. The further we go, the less real pain is, simply because our brain can't process and mirror it.

Animals look a bit like us and show similar pain reaction so we can emphasize somehow. But for everything after plants, we are getting too far. And then, we can introduce computers: can an AI feel pain? How advanced does it have to be if it does? Maybe it is all programs. Unit tests must hurt...