A barrel contains a mixture of wine and water in the ratio 3 : 1. How much fraction of the mixture must be drawn off and substituted by water so that the ratio of wine and water in the resultant mixture becomes 1 : 1 = ?

Answer: Option B

Original ratio of the mixture 3:1
Taking out the mixture actually means just taking out the wine because the water anyways is going to be added back.
if we remove 1 part of wine, it makes the ratio 2 : 2 ( 1 part water is added to keep the volume constant )
so, what we have actually done is remove 1 part wine from 3 part Wine.
i.e. $$\frac{1}{3}$$

So, $$\frac{1}{3}$$ mixture drawn.