I don't think the G#5 would be a problem for Kelly, but she wouldn't be able to sing this song convincingly as it's not her type of music, I feel like. I feel like Christina could sing it more convincingly than Kelly, even if she were to strain on the G#5.

Christina has never even hit a note as high as Ariana does in the prechorus so no, youre wrong

This exaxtly. Christina would slay this song very hard, but she would strain on that only note. Its not an easy one to pull off everyone.

Xtina wouldn't even be able to phonate the G#5's though, I'm not shading but she hasn't gone up to an F5 live in god knows how long, and a G5 in studio either. Kelly probably wouldn't be convincing as well due to the genre, that's true, and she can definitely sing the hell out of G#5's, but I just can't hear her doing it like Ariana tbh.

Even though Kelly really isn't a part of this thread, neither her or Christina would need to sing it exactly like Ariana. Honestly, even though I hate Christina's technique and much prefer Ariana as a vocalist and an artist (currently), Christina's instrument (albeit misused) has more flare to it. With that in mind, she's going to have to reinterpret the song.

I think she could sing the song, but not the same way Ariana does, obviously. Christina has an ability to make notes seem higher than they actually are (this isn't supposed to be shade) and I'm sure she could use that to her advantage in this case.

Absolutely not.

Not even in her prime could Xtina hit a F#5 regularly.

Ariana hits it in every prechorus and then hits a G#5 at the end of the song.

Sorry, I love Xtina, but the song is out of her range.

There's no way on earth Christina can sing "Problem".
She doesn't have that much control of her voice above a high C, so I can't imagine her consistently hitting those G#'s