We need to weaken the argument hence we need to establish that something that the conclusion is not the case.
Clearly, The author concludes that the so-called "suicide wave" following the stock market crash in October 1929 was not such a big deal.
To support this conclusion, the author states that the suicide rates in October and November of 1929 were lower than the rates for the months before the crash.
Now, We need to weaken that argument, as in we want to look for an answer choice that suggests that suicide rates in those months were in fact unusually higher. The trick is compared higher to what? As the author has stated that numbers of Oct & Nov are no big deal compared to other months in 1929, but if we establish a comparision would be with the suicide rates in Octobers & Novembers of the surrounding years. This makes for a better comparison because it eliminates other seasonal factors that might come into play when analyzing suicide rates throughout the year.
Choice (A) : This is clearly Out of Scope.
Choice (B) : This would actually support the argument.
Choice (C) picks up this distinction. When viewed in comparison with the Oct-Nov data, 1929 does seem like a high suicide period. Voila, This weakens the case.
Choice (D) : This would actually support the argument.
Choice (E) : This looks interesting but does affect the argument in either way. Even if we know that suicide rates are expected to be different in different seasons, but that does not tell us whether the results in 1929 were unusually higher or lower.