In 1992 outlaw fishing boats began illegally harvesting lobsters from

Understanding the argument -
In 1992 outlaw fishing boats began illegally harvesting lobsters from the territorial waters of the country of Belukia. Fact
Soon after, the annual tonnage of lobster legally harvested in Belukian waters began declining; Fact
in 1996, despite there being no reduction in the level of legal lobster fishing activity, the local catch was 9,000 tons below pre-1992 levels. Fact
It is therefore highly likely that the outlaw fishing boats harvested about 9,000 tons of lobster illegally that year. - Conclusion

There are two things happening
1. Outlaw fishing boats begin illegal harvesting. (X)
2. In 1996, the legal catch was 9000 tons less. (Y)

The conclusion says X was the reason for Y. X causes Y.

Let me share another more straightforward argument to make it clear
1. Alcohol sales increases in the city.
2. Number of accidents increase

We say the increase in liquor sales leads to an increase in accidents.

Do you see that the author has taken two events and concluded that one caused the other?

So, what assumptions do we make when we convert two events, X and Y, into a causal argument? We assume that
1. It is not a coincidence
2. There is no other cause.

Are you with me so far? If no, then read again. If yes, then let's proceed with our argument.
1. Outlaw fishing boats begin illegal harvesting. (X)
2. In 1996, the legal catch was 9000 tons less. (Y)

What will be the assumptions for our argument to be valid?
1. Outlaw fishing boats begin illegal harvesting (X), and the reduction of legal catch (Y) was not coincidental.
2. Nothing other than the Outlaw fishing boats or illegal fishing (X) could have caused the reduction of legal catch (Y).

If you have understood so far, then option elimination is comparatively easy.

Option Elimination -

(A) The illegal lobster harvesting was not so extensive that the population of catchable lobsters in Belukia’s territorial waters had sharply declined by 1996. - This is trying to eliminate other causes. What other cause? That the population of lobsters in 1996 had not gone down. Because if this is true, then our conclusion shatters.

(B) The average annual lobster catch, in tons, of an outlaw fishing boat has increased steadily since 1992. - If you don't read well (sorry, but that's mostly the case), this option talks about "ONE FISHING BOAT." But GMAT knows that under pressure, we read fast and can miss this. And we do; this is their trap. And if we miss reading that properly, this is a good option. But we are not concerned about one boat, and this in no way explains the causal relationship. But this was an innovative way on GMAT's part to create a fundamental deception - make things singular and slip them in. Distortion.

(C) Outlaw fishing boats do not, as a group, harvest more lobsters than do licensed lobster-fishing boats. - This comparison is not relevant to the causal relationship that the author has created. Out of scope.

(D) The annual legal lobster harvest in Belukia in 1996 was not significantly less than 9,000 tons. - This option is created by just putting some familiar words from the argument and somehow putting them in a sentence. Again, it's a good trap for readers who don't read properly. And for readers who read slightly less than the best, B is a good trap. Let me explain.
Say the catch before 1992 was 100,000 tons.
1996, the catch was 9000 tons less, translating to 91,000 tons.
This option says the legal catch was less than 9000 tons. (our 1996 catch is 91,000 tons, so this is in no way linked) but it took that 9000 tons from the argument and created a deception, knowing people tend to read fast on tests and tend to miss. Distortion.

(E) A significant proportion of Belukia’s operators of licensed lobster-fishing boats went out of business between 1992 and 1996. - How does it even matter? We are talking about 1996, and this option is talking about between 1992 and 1996. Moreover, the argument says, "despite there being no reduction in the level of legal lobster fishing activity," Out of scope.

Learning - Read carefully