The higher the average fat intake among the residents of a country, th

This problem does a great job of setting up a Correlation vs. Causation logical flaw, so that's a big lesson you should take away (even outside of the C vs. D debate). The premises show that fat intake and cancer incidence occur together (correlation), but then the conclusion makes an inference about causality (if you reduce your fat intake you'll reduce your likelihood of cancer incidence, basically assuming that fat intake is the reason for cancer).

When you see Correlation vs. Causation logical gap (X and Y occur together, so X causes Y), the most likely correct Weaken answers will be:

1) X doesn't cause Y; instead Y causes X --> the opposite of the conclusion is true

2) Actually Z causes Y (and maybe also causes X) --> there's an alternate cause of the effect (and maybe of both effects)

Here choice (D) is written to hit that #2 type of correlation/causation Weaken answer - environmental pollution is given as the alternate cause.

So that's the lesson to take from this one. For anyone thinking "hey that's a pretty big assumption to make that pollution and cancer are connected" I think I'd agree with you, but keep in mind that official questions are so carefully developed and statistically researched that they'll (almost) always get it right (well, at least if they're the non-experimental ones that count toward your score). So here even if you don't think D is a perfect fit, if you see how it fits the Correlation/Causation template you've learned the lesson and are ready to apply it to future problems.