Se renseigner sur le propriétaire d'un ou plusieurs objets: J'utilise le mot interrogatif whose.

Look, there is a pen on my desk!/ Whose pen is it?/Whose is it ?   It's Laura's (pen)!
I've got two rulers here!/ Whose rulers are they?/Whose are they? they're Ken's (rulers)!

It's=
They're=
It isn't=
They aren't=