To answer this question we have to find (among the options) the amount that represents half the amount of water used.
Since the ratio of water to vinegar is 2:1, half of the amount of water will be used of vinegar.
In this case we have to find the answer that represents half a gallon.
That answer is 2 quarts. 2 guarts are 0.5 gallons, it means they are half the amount of water used.
It means that the answer is b. 2 quarts.