X/Y = (18 - 12)/(12 - 10) = 6/2 = 3/1

St1: X = 3/4(X + Y)
4X = 3X + 3Y
X = 3Y --> X/Y = 3/1 --> We know this from the question itself
Not Sufficient

St2: X + Y = 16
3Y + Y = 16 --> Y = 4 and X = 12
Sufficient

Answer: B

From the question prompt, the ratio of Solution X to Solution Y in the combined solution is (18 - 12) : (12 - 10) = 6:2 = 3:1.

1. Solution X comprises 3/4 of the combined solution
This statement provides no extra information than what the question prompt already provides. It's a Tautology!
So statement 1 is insufficient.

2. The combined solution is 16 liters.
Since we know the ratio of Solution X to Solution Y already, we can find the quantity of X or Y in the combined solution if the quantity of the combined solution is given. No need to calculate the answer. Just know that you can calculate the answer!
So statement 2 is sufficient.

Answer is B.