Standing waves can only occur in a medium that iscontained.liquid.gas. two-dimensional.solid.

Standing waves can occur in any medium that is contained, whether it is a liquid, gas, or solid. They are not limited to a two-dimensional medium. The key factor is that the medium must be contained or bounded in some way. This is because standing waves are the result of waves that are reflected back upon themselves, creating a pattern of constructive and destructive interference. This can only happen if the waves have boundaries to reflect off of.