Question

1 It's the city in which/where the worst environmental problems
in the country exist.
2 Everything that/what they told us about Christmas in Ukraine was interesting.
3 This is the place which/where archeological finds started.
4 Anything that what he says is bound to be rubbish.
5 That's the town which/where I was born.

Answer 1

1.which
2.that
3.where
4.that
5.where