WeakenX-CE. The correct answer is (D).
This stimulus begins with the author's conclusion: garbage dumps do not harm wildlife. One way to tell that this is the central belief is that the remainder of the information is classified as "evidence," meaning given to support a view.
So what evidence are we offered to conclude that garbage dumps do not harm wildlife? The single example of baboons on the Masai-Mara reserve in Kenya that use local garbage dumps as a food source. Members of this baboon troop mature faster and have more offspring than do baboons on the reserve that do not feed on garbage.
This may seem at first reading like a somewhat convincing argument—after all, if garbage-consuming baboons mature more quickly and reproduce more readily than their garbage-free neighbors, isn't it reasonable to conclude that they're healthier?—but when we start to really scrutinize the assumptions made here problems quickly arise.
A few points on this.
First, comparisons are tricky. To make a truly legitimate sweeping comparison, like overall fitness, between two things, like baboon groups, you need to know a lot of information about them. They need to be of the same type, for instance, otherwise what's the point of an A/B measure?
Second, the conclusion is that "garbage dumps do not harm wildlife." That's an incredibly broad belief on three counts: dumps in general (rather than just garbage as food), no harm whatsoever, and all wildlife. For the first, we can't classify dumps safe merely because it might be safe to eat their contents. Many, many other hazards could arise from dumping garbage wholly unrelated to its consumption. For the second, multiple measures of health, certainly more than just two superficial physical observations, would be needed to conclude something so generalized as "no harm." And for the third, can you really draw a conclusion about wildlife on the whole from some information about a single troop of baboons? (hint: you cannot)
With that in mind, it seems clear that the implied causality between eating garbage and improved fitness cannot be fully trusted, nor can the jump from either eating garbage to general dump safety or from baboons to all wildlife.
So this argument is actually riddled with issues, making it ideal for a Weaken-Except question stem. We'll have four (incorrect) answers that undermine the conclusion, and a single answer (correct) that does not—it may help, or it may be irrelevant; it simply can't weaken.
Answer choice (A): This is, in my opinion, the most interesting of the wrong, weakening answers. To many it seems irrelevant, but think about what it's telling you: if the two baboon groups being compared to establish one group's superior fitness are in fact different species, then the comparison itself becomes meaningless! Maybe the species that eats garbage is supposed to mature faster and have more offspring. Noting these differences is only useful if they're unexpected or unusual, but with two different species there's no way to tell if the maturation/reproduction "success" is actually success, totally expected, or even less/lower than it should be.
In other words, the possible effects of garbage consumption can't be known at all if the comparison it's based on is irrelevant.
Answer choice (B). This answer, along with the other two wrong answers not yet discussed, presents a direct attack on the health of the garbage-eating baboons: their life expectancy is significantly lower than that of the other group. This clearly weakens, since it shows that there may well be a harm to eating garbage after all. In essence it shows the cause (eating garbage) possibly leading to an opposite effect (diminished health).
Answer choice (C): In a near-copy of answer choice (B), we have another negative health statistic for the garbage-eating troop: cholesterol levels dangerously higher than those of the other baboons mentioned. Once again, it's a cause leading to a different/conflicting effect, severely weakening the argument.
Answer choice (D): This is the correct answer. Saying that a population of hyenas living near garbage landfills has doubled has absolutely no harmful effect on this argument, primarily because (1) we don't know anything about the hyenas' behavior (do they eat the garbage?), (2) we don't know if the doubling population is normal, healthy growth or if it should have increased even more, and (3) we can't assume any connection between that growth and the landfill itself.
If anything, one could possibly argue that this answer strengthens the argument slightly—the garbage dump they live near clearly hasn't reduced their population size, so maybe it is as harmless as the author believes—but regardless it should be clear that this answer doesn't weaken the conclusion. Because it's the exception here it is correct.
Answer choice (E): If you understood the negative effects of (B) and (C), (E) should present no problems. If birth defects have doubled for the baboons on the reserve since the landfill opened, then it increases the likelihood that the dump does indeed cause some harm to wildlife. It's another instance of the cause likely tied to an effect that contradicts the author's claim, and thus weakens the argument.