Right answer is (b) Concentration by Mg(NO3)^2
For explanation I would say: Magnesium nitrate solution containing 70-75% Mg(NO3)^2 is fed to a dehydrating tray tower along with dilute HNO3 from the absorption system. The salt solution act as an extractive distillation agent, removing water at 100 C and the solution is again re- concentrated by evaporation.