The domain is the set of all the values of X therefore it would be D.
So hmmm let's say, the pitcher has "x" cups when full
so, it was (3/4)x then 4 cups came out, went down to (2/3)x
now, if we subtract (2/3)x from (3/4)x, their difference will be the 4cups that were served
thus

and surely, you'd already know what "x" is