Ken's, потому что whose переводится как "чей", а Ken's - это притяжательная форма имени
There are books on the table
You are happy,aren’t you?
He has already written his essay,hasn’t he?
They won’t go to the circus,will they?
He was at home yesterday,wasn’t he?
They have been playing football for 2 hours,haven’t they?
She isn’t riding her bike now,is she?