D is incorrect. Upper limit on the size of the shrimp which means that they cannot catch shrimps bigger than that. That is what the statement literally means and even if it is assumed to mean upper limit on the weight of the shrimp harvests, then it does not limit the number of recreational catchers which makes the plan useless.

Large-mesh nets can reduce or nullify chances of catching shrimps and instead only big fishes would get caught.

nileshdalvi has given a good explanation for why the answer is C and not D. Let me add to it a bit.

The problem is that the recreation fisherman are catching the shrimp while those shrimp in the estuaries while they are still very small. So the problem is that they are catching small shrimp before those shrimp make it to the ocean. Answer choice D mentions an UPPER LIMIT on the size of the shrimp recreational fisherman can catch. But this would not help. What is needed is a LOWER LIMIT on the size of the shrimp. In other words, this would keep the recreational fisherman from catching the really little guys. So D is not much help.

Now look what C does. By requiring the use of nets with larger mesh choice C actually does put a LOWER limit on the size of shrimp that will be caught, because smaller shrimp will drift right through the holes in the net if it has a larger mesh. So C really does what we need for it to!