It seems like Alex would agree with A, however "most" has a very specific meaning and Alex never says "most" shrimp farms are abandoned. On these disagreement questions, we should try to clearly determine the authors' positions on each answer choice. If we are uncertain how an author would react, it is probably not the right answer.
For the sake of argument, let's assume Alex agrees with A. Can we say that Jolene disagrees with it? No, we cannot. Jolene might agree that most shrimp farms are eventually abandoned. She is only trying to argue that they aren't quickly and carelessly abandoned. For these reasons, A is incorrect.
We can say with confidence that Alex agrees with answer choice B. He says that investors make a quick profit and then bail. Does Jolene disagree? Yes. She says that shrimp farming is not such a quick, simple process. She says that a good shrimp farm takes a long time to build, and owners are not eager to abandon them. This is why B is correct.