Hello Shylock, - :"live" is an adjective meaning ‘active’ or ‘living’ in a figurative sense as in a live show or a live experience. - "living" as an adjective relates more to "living creatures" so it is better to use it in your example "living fishes" specially that the sentence is opposing them to "dead ones". Thus I am more likely to choose "A" in this specific context.

The whole sentence has incorrect grammar. It should look like this: "___ fish are usually more expensive than dead ones, because it is not easy to keep them ____ ." The words "living" and "live" (pronounced to rhyme with "dive") BOTH mean the same thing -- "not dead". The word "living" tends to be used of people, trees and ideas. The word "live" tends to be used with animals and electronics (or electronic broadcasts). So, A.) and B.) are both technically correct. But choice B.) is better, because the sentence is about animals.