What's the grammar of this sentence?
'Co to za...' is used as ' What kind of ___ is this/that' when you choosing from a wide set of possibilities.
When you have few possibilities to choose from you would say 'Który to zapach?' - 'which smell is it (or scent in this context, like perfumes)'.
It's worth pointing that this automated pronunciation is quite messed up. "z" in "zapach" shouldn't be softened, there is no such phone in Polish.
what kind of smell is this ? is not accepted, why not?