Strictly speaking it’s none of them, because those are idealized perfectly symmetrical spaces with no matter in them, only dark energy, and our universe (happil

Strictly speaking it’s none of them, because those are idealized perfectly symmetrical spaces with no matter in them, only dark energy, and our universe (happily) has matter in it :). But it’s very, very close to flat, except not quite perfectly flat, and the best observational evidence leads us to believe that if you neglect the matter and think only about the dark energy part, we’d actually be living in a de Sitter spacetime. The quantity that measures this is called the cosmological constant. It’s zero in flat space, positive in de Sitter and negative in anti-de Sitter. It turns out from measurements that our cosmological constant is positive but outrageously small, tiny compared to anything else we know about in physics. This is puzzling because we would love to relate it to something we understand already but it’s hard to arrive at a result so small working with quantities that are considerably larger. So there’s an interesting open question about why it is what it is.