The correct answer is:
1 gallon of white paint.
Explanation:
It takes 1/4 of a gallon of white paint to lighten 2 gallons of brown paint.
He bought 8 gallons of brown paint; this means he has 8/2 = 4 groups of brown paint he can lighten.
Each group of brown paint takes 1/4 of a gallon of white paint; 4(1/4) = 1 gallon of white paint total.