The answer is D: sugar/flower. Think about it as fractions. Milk to Flower is 3:6 which equals 1:2. Sugar to milk is 2:3 which equals 2:3. Oil to sugar is 1:2 which equals 1:2. Sugar to flower is 2:6 which equals 1:3. 1/3 is clearly the smallest fraction so D is the right answer.
Hope you understand this!
