Conclusion: the environmentalist's claim should be dismissed without further consideration.
Why? Because even though the environmentalists have argued that the proposal would interfere with bird-migration patterns, these same people have raised environmental objections to virtually every development proposal brought before the council. The argument poses that this indicated that the environmentalists aren't actually concerned for the environment, but are actually just trying to propel their antidevelopment and antiprogress agenda.
So, what's the issue here? Well, we are concluding that the claim of the environmentalists should be dismissed, not because of the content of the claim, but because of their purported motives.
Regardless of motive, dismissing a claim should be made on the actual claim, as opposed to any peripheral issues surrounding the claim, i.e. not on the motives of a specific group.
This is what answer choice (B) points out: "rejecting the conclusion of an argument on the basis of a claim about the motives of those advancing the argument."
If you have any specific questions on another answer choice, please feel free to clarify.
Hope that helps! Please let us know if you have any other questions.
