Kterou z těchto sušiček chcete?
I put the same sentence without the "ty". Is the "ty" here just to stress that it's "you", or does it have some specific other role?
It is there to stress "ty" so that we are asking which one of them do YOU want. I want this one, which one do YOU want? Já chci tuto, kterou chceš ty?
Seems to me we cannot know where the stress lies : on the YOU (as you read it) or on the WANT ( as I read it), in which case surely the answer without "ty" should not be marked wrong?
As always, if any answer of yours was rejected, you must report the complete sentence. Use the reporting button.
why isn't it "Kterou jedna z ..."?
You can't translate every word when learning foreign languages. English "which one" is just "který" in Czech.
The English would also be perfectly correct without the 'one'.