The Sadducees were the party of the priestly class, the spiritual nobility. They were more political than religious. As adders flee from an approaching forest fire, so did the Jews flee from the coming Judgement. John's question is rhetorical, with the stress on 'who'. Produce fruit in keeping with repentance .
Fruit trees are useful only if they bear fruit.
We have Abraham as our father . But John rejected this idea and said that descent from Abraham was no substitute for conversion and good works. Out of these stones God can raise up children for Abraham . It is therefore possible that the stones are an image of the Gentiles. The axe is already at the root of the trees.