(A) must be assumed. Remember, our explanation is that these environmentalists are actually cynical, i.e., they don't really care about the enviroment. They are just anti-progress. Well show me at least one development proposal which would not have posed a threat to the environment but which they opposed anyway. In other words, take the environmental variable out of the equation and show me that they still oppose it. Afterall, the claim is that they are just anti-progress, right?

And imagine if we couldn't. Imagine if the only developmental proposals they ever opposed were the ones that posed a threat to the environment. Well, that would be ruinous for our cynical explanation. It would strongly favor the sincere explanation. That's why (A) is necessary.

(B) points in the right direction, it’s just too strongly worded – change “always” to “sometimes” and you’re golden.

The author does need “disguising motives” to be a thing that sometimes happens. If it’s a thing that never happens, the argument is screwed.

But the author doesn’t need it to happen every time. It’s totally fine if some anti-development environmentalists somewhere else aren’t trying to disguise their true motives – all that matters to the author is that these environmentalists are trying to disguise their true motives.

This would vaguely strengthen the argument if it were true – it would be a helpful principle to conclude the environmentalists are anti-progress – but it’s far too strong to be necessary. Change “anyone” to “some people” and you’re getting closer.

This would be relevant if the author’s conclusion were “the council should therefore approve the Golden Lake Development proposal.”

The author’s actual argument, however, is about whether the environmentalists are in fact motivated by environmental concerns. Other considerations that the council might or might not consider are irrelevant to this much more narrow topic.

(E) points in the right direction, it’s just too strongly worded – change “almost always” to “sometimes” and you’re golden.

The author does need it to sometimes be true that people say one thing when they’re secretly concerned about something else. If no one ever does that, the argument is screwed.

But the author doesn’t need everyone who opposes development projects to always have secret ulterior motives. It’s totally fine if a few environmentalists somewhere else are being genuine in their environmental objections – all that matters to the author is that these environmentalists have secret ulterior motives.