Whose toy is it?
The toy is his. / It's his.
Whose wallet is it?
The wallet is hers. / It's hers.
Whose car is it?
The car is theirs. / It's theirs.
Whose phone is it?
The phone is his. / It's his.
Whose bike is it?
The bike is theirs. / It's theirs.
Whose house is it?
The house is theirs. / It's theirs.
Whose ball is it?
The ball is his. / It's his.
Whose cake is it?
The cake is hers. / It's hers.
Whose TV is it?
The TV is theirs. / It's theirs.
Whose computer is it?
The computer is hers. / It's hers.