Premise - In parts of the Caribbean, the manatee, an endangered marine mammal, has long been hunted for its meat. 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.

Conclusion - 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.

Anything which weakens the conclusion is our answer

A)Many 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. (Wrong as it is already given in the passage that Tourist interest is high)
B) Recovery of the species would enable some hunting to continue without putting the manatees’ survival in jeopardy again. (Neutral statement)
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. (Rather than weakening it strengthens the conclusion, eliminate)
D) There would not be enough former manatee hunters to act as guides for all the tourists who want to see manatees. (Not relevant)
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 (This is our answer, if the incentives the hunters receive as tourist guides is less or the trouble is more they might not switch the occupation)