The correct answer is C) Correct the readings to a reference temperature; reference temperature should be recorded in prior readings history. The reason this is the correct answer is that insulation resistance values are temperature-dependent. To accurately compare current readings to prior readings, the values need to be corrected to a common reference temperature, which should be recorded in the historical data. This allows for a logical comparison of the readings over time, accounting for temperature variations that can affect the insulation resistance measurements. The other answer choices are incorrect because: A) Using a different megohmmeter would not ensure consistent measurements, B) and D) the duration of cranking the handle on a hand-crank megohmmeter does not directly affect the accuracy of the reading, as the important factor is allowing the reading to stabilize.