Here, you're mixing scores 85 and 90 with weights X and Y, respectively. You are asked for the ratio of X to Y.
There's a quick way to work mixture problems of all kinds. Write the two components of the mixture on the left. Here, they are 90 and 85. (I usually put the larger one on top.) Put the mixed value in the middle, and form differences along the lines of an X, as shown. The numbers on the right give the relative contributions of the constituents at the same level in the diagram. Here, the ratio of X to Y is shown as 2 to 3.
For some mixture problems, you need to know the proportion of the constituent to the whole. In that case, add the ratio values to get the "whole". For example, here the X class students make up 2/(2+3) = 2/5 of the whole number of students.
For your problem, X/Y = 2/3, corresponding to selection D.