Why is C the right answer?

If I understand the solution to this problem correctly, the issue here is to challenge Oscar's assertion of "single most important" factor. But we are told in many, if not most stimuli, that we are to "presume" that the premise(s) in the stimulus is correct. So, to be be clear, in an "Errors in Reasoning" Question, are we "free" / supposed to challenge the truth of that premise, particularly when it is a Point as Issue structure? Moreover, in her explanation, Naz said a combination of "smaller factors" could conceivably contradict Oscar's premise. That seems vague. Thank you for your help.