According to the explanation in my book, number 2 'what' is grammatically incorrect, so it should be changed to 'that'. That is, it-that cleft structure
But, I think 'what' is not necessarily incorrect. What about this? This can be possible
ex) What preserved their culture legacy for descendants was the dense jungle
= It was the dense jungle what preserved their culture legacy for descendants
Is there any reason why 'what' is incorrect ?