+0
If the pen isn’t yours, ______ ?
A. who else's can it be B. whose else can it be C. who’s else can it be

Which is right?
+2
The possessive '-s' is added to the 'else', not the pronoun:

Who else's pen is it?
Is it anyone else's?
I think it's somebody else's.
+1
apostrophe-s ('s) is added to the very end of the whole expression being considered possessive.

the King of England's crown
John and Mary and the kid's home
who else's problems
someone other than Alice's car
a friend of mine's book

Not sure about these, however:

John and I's agreement?
John and me's agreement?
John's and my agreement?

I've heard the first two. I'm certain of it.

Maybe Mr. M or Mr. P can help us on this point.

Emotion: smile
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Comments  
It's John's and my agreement. I had to look that one up some time ago. If you get it wrong, don't worry; who else's going to know?
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Who else's could be
qingqing If the pen isn’t yours, why are you in it?

Emotion: rofl

CJ

whose else

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