Assuming total marbles = 100
& Cost of each marble = 1 rs.
∴ To buy 100 marbles designer need to spend 100*1 = 100 rs.
After increasing price cost of 100 marbles = 125 rs.
Now, at 100 rs. the designer can buy $\dfrac{100}{125}*100$ = 80 marbles
the designer decided to reduce equal numbers of marbles of each colour to keep the total cost unchanged.
∴ (40 - x) + (25 - x) + (20 - x) + (15 - x) = 80
4x = 100 - 80
x = 5
Percentage of blue marbles in the new design = $\dfrac{40 - 5}{80}*100$
= $\dfrac{35}{80}*100$
= 43.75
Hence, the answer is option C)