Which kind of rock is soluble in water?

A terrible question. Limestone is NOT soluble in water, just as the description explains. It is only soluble when water has acid added to it, at which point it is an aqueous solution, not pure water.

So many limestone caves where I come from all caused by underground rivers.

Wrong answer - limestone does NOT dissolve in water, e.g. two separate layers are formed, which apparently is not a real solution.