The correct answer is
(D) recognise that two different methods of investigation can yield identical results.
Reasoning for the correct answer: The argument concludes that there's "no need to look further for an explanation" simply because the studies used different methods of investigation. This assumes that different methods will necessarily produce different results. However, this ignores the possibility that well-designed studies using different methodologies could still arrive at the same conclusion if they're measuring the same phenomenon accurately. The fact that the studies used different methods doesn't automatically explain why they reached contradictory conclusions about the same time period.
Reasons for eliminating the incorrect options:(A) is incorrect because distinguishing between government and university research is irrelevant to the logical flaw. The source of the studies isn't the issue; the problem is assuming that methodological differences alone explain contradictory results without considering that different methods could potentially yield the same results if both were valid.
(B) is incorrect because the argument doesn't confuse methods and purposes of investigation. The flaw is about assuming different methods necessarily lead to different results, not about mixing up methods with purposes.
(C) is incorrect because the argument doesn't need to recognize that one study was improperly conducted. The argument's flaw exists regardless of whether either study was properly conducted. The issue is the assumption that methodological differences automatically explain contradictory results.
(E) is incorrect because varying economic conditions aren't relevant to the logical flaw in the argument. The argument is about explaining the differences between study results, not about explaining why workweeks change in length. This answer introduces an external factor not addressed in the original argument.