First, the argument conclude that because of a new tax policy, profit margin will be higher in seafood store within the city than outside, and keep in mind that we want the most USEFUL answer - the answer give us the most relevant information to evaluate the argument

(A): Option A tell us there are more store within the city rather than outside, this only tell us it's likely that total profit of store within the city will be higher, but it gives us no information about the profit margin

(B): Option B offer us similar information like option A, give us only the indication of number of stores within and outside the city, if the number of store inside the city after relocation is higher than outside the city, it's basically information of option A again, so this definitely cannot be the answer.

(C): Option C tell us price of fish will fall, but since both type of store get fish from the same source, they will be affected similarly, so we can't determine anything using this information

(D): Option D is basically information from the passage, so it's not relevant

(E): Option E tell us that price will be different, combine with the difference in tax policy, now we got 2 elements to work with (difference in selling price and difference in tax), therefore option E will be MOST USEFUL

Answer: E