B. 8%
Feedback When 10 litres taken out, quantity of mixture left =50-10=40, so
milk left = 90/100 *40 = 36
And water left = (10/100)*40 = 4
now 10 litres of milk poured in, so there is no change in quantity of water, and total mixture again becomes 50l
so water now is 4/50 * 100 = 8%