You can use the the pick numbers method.
Let say 8 dollars buys you 8 ounces, in which each ounce makes 2 cups.
So that costs 1 dollars per ounce, which will make 2 cups.
This is the same as .50 dollars to make 1 cup.
Now plug in 8 for d, and 2 for c, and see which answer gets you .5.
A. (8)/(8x2)= .5
A is your answer
(BTW if you use this method, you need to make c an integer greater than 1.)