The stimulus is basically telling us that there are some symptoms of mental illness that are affected by various organic factors. However, in looking around the globe, there's a large variation in the incidences of these symptoms. Because of this, the argument concludes that the organic factors that are actually affecting people vary throughout the world.
So we know that symptoms vary a lot from culture to culture. While it's true that organic factors varying could be the reason for this, it's possible that they're not the reason. Perhaps some culture simply reveals certain things more than others do. The argument is ignoring the fact that it's possible that the mental illness symptom incidences are caused by things that aren't organic.
(C) says, "fails to consider the possibility that cultural factors significantly affect how mental illnesses manifest themselves in symptoms"
(C) describes a type of non-organic variable that might be causing the variation in mental illness symptoms, and this is something that the argument ignores. This is why (C) is the correct answer.
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