i think it's d.

Quiet888, the color is the clue you should pay attention to when evaluate the argument.

Chase of cause:

Mosquitoes...delibitated --> rub

To evaluate this argument, you should consider whether or not Mosquitos is the only cause make Capuchin rub his fur?

I hope you can understand D under this brieftly explaination!
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Premise:
Capuchin monkeys in Venezuela often rub a certain type of millepede into their fur. Secretions of these millipedes have been shown to contain two chemicals that are potent mosquito repellents, and mosquitoes carry parasites that debilitate the capuchins.

Premise:
The rubbing behavior is rare except during the rainy season, when mosquito populations are at their peak.

Conclusion:
Therefore monkeys probably rub millipedes into their fur only because doing so helps protect them against mosquitoes.

We are looking for information to help us evaluate the conclusion. So we need information that can help up determine WHY the monkey rub millipedes into their fur.

A. Whether the chemicals provide protection for the millipedes is irrelevant. We are only concerned with why the monkey rub the millipedes into their fur.

B. Again out of scope, we are only concerned with these monkeys in Venezuela.

C. Also out of scope, only concerened with monkeys

D. Correct. One of the premises is that the monkeys only rub them into their skin during the rainy season when the mosquito population is highest. They use this premise to arrive at the conclusion that this is only done to protect against mosquites. But adding this additional information that millipedes are only available during the rainy season cast doubt on this logic. It could now be that the only reason they rub them into their skin during the rainy season is because this is the only season they're available. It may just be a coincidence that the millipedes are availabe only during the rainy season when their needed and the monkeys might do the same if millipedes were available in non rainy season.

E. Out of scope, we are only concerned with monkeys and millipedes.

Sorry, but according to my source, OA is E.

If capuchin monkey uses other insect to repel mosquito, it may rub the miliepede into its fur for other reason (for example, because doing so excites the monkey
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Noboru, I think the answer is D. E seems to be making a lot of assumptions. Could you please let us know what is your source ?
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Yup, please let us know that; I always felt E is a bit tangential. The variance test suggested by powerscore CR Bible brings out D as answer.
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i thought D too but after knowing the answer i agree with E...
the conclusion is that this stuff is used majorly in rainy season and its ONLY function is to repel mosquitos.

While we know that mosquitos population is at is peak in rainy season, but it is very much possible that the monkey can contact the desease from a mosquito in a non-rainy season.

As E suggests, what if the monkeys used other chemicals during the non-rainy months to protect itself from mosquitos and uses millepede in rainy season because it can do 2 things
1. repel mosquitos
2. protect the monkey from getting wet in the rain or from catching a cold

then the conculsion that the millepede is used ONLY as a mosquito repellent is incorrect!!

D is a close contender, but as suggested by E, if there were other chemicals that could repel mosquitos, although the millepede is available only during rainy season, the monkeys MAY use millepede or MAY use a different chemical they have been using all year long!

this again draws my interest in answering:

premise, C monkeys use millipeede's to rub their fur.

they may do so to protect themselves from mosquitoes, dat to durin raining season.

we find out from options :

A. OOS
B. OOS
C. Even if they rub, we dont have to do anything with it
D: hold it, not because i understand it but its due to i dint understand it.
E: out of box

so we are left with D.

analyse this, if millipedes are available in all seasons and if they use only in rainy season.