allegation and mixtures

Well, 30% of 8kg will be 0.3*8 = 2.4, which will be the amount of liquid X in your initial batch of solution. You get that 2 kg of water evaporate, which means that you now have 6 kg of solution containing 2.4 kg of liquid X.
Adding 2 kg of solution Y means adding a mixture made of 0.6 kg liquid X (30% of 2) and 1.4 kg of water (70% of 2).
This means that the final weight of the solution will be 2 + 6 = 8 kg, while the final weight of liquid X will be 2.4 + 0.6 = 3 kg. The percentage will therefore be x = 3 * 100/8 = 37.5% or 75/2 %.
Answer C

Original solution ---> 30% (0.3) concentrate of 8kg = 8*0.3 = 2.4kgs
Removed--> 2 kg of water (water has 0 kg of liq Y)--> 0 * 2 = 0
Added --> 2kg of solution--> 2* 0.3 = 0.6 kgs of liq y
Final solution = 8 kgs = 8*X(conc of liq Y) = 8x

Final solution = original - removed + added

8*x = 2.4 - 0 + 0.6
x = 3/8
% of x = 3/8 * 100% = 75/2%

C is the answer