Hi Ed,
I'd like to add a little here if I can as this question was tricky for me as well. You're exactly right that at first blush, D seems pretty good. In fact, I almost changed my answer to D, but time expired and I kept A.
In retrospect, it's a lot clearer to me now. I agree with your point and Robert here. The passage doesn't even mention treaties until line 51, so how can the empirical studies discussed in line 29 be referencing treaties as D states? I think in a lot of the questions in this passage, the writers were trying to test you on that distinction between the treaties and formalized negotiations vs amorphous and non-codified principles (also the difference between those principles and how they are applied).
To take it a step further though, the writers really modulated the difficulty of this question by disguising and complicating the language of the correct AC. This question could've been a lot easier. My pre-phrase for the AC was something along the lines of
there is a dearth of empirical investigation into the extent of how often harmful pollutants cross international borders of countries that claim to abide by these international norms . The correct AC here kind of takes a little translating, but actually matches up with the pre-phrase quite nicely. There is a dearth of investigation into "The extent to which actions of nations..." aka extent to which nations pollute across borders, "...conform to principles of customary intentional law" aka conform to the claim that they do not pollute/abide by norms.
The question also hinges on your understanding of the word dearth which adds to the difficulty.
Hope that helps