The questions asks how much more water is in J than K, which means you need to know quantity of water, not percent. You have percent capacity, which means one of the answers is going to have to give you quantity or it's unanswerable. I think that is where you said you got confused. Answer (1) gives you measureable capacity (you now know that J holds 6 and K holds 5) but doesn't tell you how full they are. Answer (2) tells you how full they are, so the answer should be C, as Lovely Baby indicated.