Ratio of Red to White marbles in Bag A : R:W = 1:3 => R:W = 2:6 =>
Ratio of White to Blue marbles in Bag A : W:B = 2:3 => W:B = 6:9

=> Ratio of Red to White to Blue marbles in Bag A : R:W:B = 2:6:9

SO Total marbles in Bag A = Ta = Rx+Wx+Bx = 2x+6x+9x = 17x


Ratio of Red to White marbles in Bag B : R:W = 1:4

SO Total marbles in Bag B = Tb = x+4x = 5x

since together, the two bags contain 30 white marbles
=>
6x+4x = 10x = 30 => x = 3
=> Red Marbles in Bag A = 2x = 2(3) = 6

So,6 red marbles could be in bag A. Answer is : 6

You have assumed that ratios of both the bags are in the same multiple i.e. x, but what if Bag A contains 17 balls and Bag B contains 10.
In this case
Bag A will contain 2*1 = 2 red balls, 6*1 = 6 white balls, and 9*1 = 9 blue balls.
And Bag B will contain 1*2 = 2 red balls and 4*2 = 8 white balls.
So multipliers are different for both the bags.

Still, Bag A will have White balls in multiple of 6 and only option that is multiple of 6 is 6 itself.