Some really excellent thoughts here! Let me see if I can tie them all together.
On
Inference questions, we must treat the stimulus as a list of trusted facts, and the correct answer choice will be fully supportable from those facts.
fmuirhea is absolutely correct that because this is a "most strongly supported" inference question, the answer does not need to be 100% mathematically provable - just extremely likely!
Inference questions are hard to predict, because the correct answer may hinge on only a small portion of the giving fact set. Here,
shaun_79 adeptly notes that two particular facts support the correct answer:
1) that poison was used 4 years ago to attempt to eliminate the pike, and
2) that fishery officials are still considering options to eliminate the pike.
While this does not strictly guarantee that the poison was unsuccessful, it sure is likely. In fact, we have to work rather hard to make up a plotline where it isn't successful (the poison completely eliminated the pike, then somehow in the space of four years, the pike was reintroduced and is now a threat....again...).
This answer is very strongly supported by the combination of those two facts!
The Unsupportable
(A) We know only that draining the lake has been ruled out, and that the poison caused the tourism economy to suffer. We have no information about what draining the lake would or would not cause!
(B) We know poison was used 4 years ago, and we know the locals got really mad, but there's no indication that this was the only time it happened. As
GeneW astutely points out above, they might have tried poison multiple times in the distant past. In fact, they might have also tried it last year! Just because it made people mad doesn't mean they won't do it again. Maybe the fishery officials don't really care if they make the locals mad.
(D) We only know they used poison 4 years ago. We're given no information at all about what they did or did not consider before choosing poison at that point.
(E) If this were true, it might be an explanation for why people might be so concerned about eliminating the pike. But we have no evidence for this. We only know that 1) fishery officials want to eliminate the pike and 2) the pike could threaten salmon and trout. There's no direct connection in any of this to the region's economy.
I hope this makes this question a bit clearer! Please let me know if I missed anything!