We know that alkenes and alkynes are unsaturated compounds. So all that we have to do here is to find out which of the above compounds is an alkene or alkyne. That will be the undaturated compound . Now out of the above ocmounds , `C_(2)H_(4)` is unsaturated (beacuse it is an alkene corresponding to the general formula of alkenes `C_(n)H_(2n) "with" n=2)`