Understanding the argument -
In parts of the Caribbean, the manatee, an endangered marine mammal, has long been hunted for its meat. Fact
Having noted the manatee hunters’ expert knowledge of manatees’ habits, local conservationists are encouraging the hunters to stop hunting and instead to take tourists on boat rides to see manatees. Plan to "take tourists on boat rides to see manatees."
Tourist interest is high, so the plan has promise of achieving the twin goals of giving the former hunters a good income and helping ensure the manatees’ survival. - Conclusion. Two goals: "giving the former hunters a good income and helping ensure the manatees’ survival."
Option Elimination -
(A) Some tourists who visit these parts of the Caribbean are uninterested in manatees and would not be willing to pay what the former manatee hunters would have to charge for boat rides to see manatees. - "Some" can be at least 2 or 2%. Ok. So, say 98% are still interested. The conclusion is still valid. Distortion.
(B) Recovery of the species would enable some hunting to continue without putting the manatees’ survival in jeopardy again. - Scope of our argument is to weaken the plan's success - what plan? Hunters take tourists. For what? To earn more and also save Manatees. So this option says, "Recovery would enable some hunting." Does this hypothetical is linked to our scope? No. Out of scope.
(C) In areas where manatees have traditionally been hunted for food, local people could easily replace the manatee meat in their diets with other foods obtained from the sea. - at best, a strengthener.
(D) There would not be enough former manatee hunters to act as guides for all the tourists who want to see manatees. - Even better. The supply of hunters is less, and demand is more. They can make good money, which is the objective. At best, a strengthener.
(E) To maintain their current income, manatee hunters who switched to guiding tourists would have to use far larger boats and make many more trips into the manatees’ fragile habitat than they currently do. - First, even to make the current income (we aren't talking about more), they need bigger boats and, even then, to make more trips, which eventually impacts the fragile habitat of manatees, thus further harming them. So, the Manatees get harmed even if they make more trips in the future; the tourists may not see manatees, which eventually means future business loss. It's a loss-loss scenario. Weakener, we need.