The argument is straightforward. Marine workers recognize that dolphin sightings increase the popularity of a beach, so they think that more dolphin sightings will lead to more popularity. However, given the limits of the information given, the argument assumes that whatever leads to more dolphin sightings will not itself counteract the positive benefit of the larger number of sightings.

To weaken the argument, we need to find a reason why that assumption is false. Choices (A) and (B) strengthen the plan's likelihood of success, while (C) and (E) are not relevant. Choice (D), however, is exactly what we're looking for. It tells us that the proposed method (further domesticating the dolphins) is not what attracts people to Nine Mile beach. (D) is correct.