Question

the claim that in twenty years time是什么从句？

Answer 1

The claim that in 20 years time 不是一个从句，因为没有谓语动词。
可以改为: the claim that is first in 20 years time. that is first in 20 years time 这就是一个定语从句。

Answer 2

They claim that in twenty years we
will have a more strong city.
这是宾语从句。

追答

They claim that in twenty years we
will have a more strong city.
这是宾语从句。