Concept: The question tests on Percentage and comprehension of question and its inputs.

Solution Y:    
    Liquid X =30%    AND   Water = 70%

     8 Kg of Solution Y is taken  and  2kg of Water evaporates from this.

=>8kg of Y  has 30%of 8kg=2.4 kg of liquid X originally in it

    2kg of Y  that evaporated had 30% of 2kg  or 0.6kg of liquid X  in it. 
    The same is added back=>0.6 kg of liquid X that evaporated is added back to Y

Q. stem- We have to calculate the new percentage of liquid X in this solution(originally it was 30%) 

               This should be the "new" weight of liquid X in the solution/ Total weight of the solution *100

              =2.4+0.6 / 8 =3/8 *100 = 37.5%  (option c)