Choice "C" is correct. Regression analysis is a statistical method that fits a line to the data by the method of least squares. It is the most accurate way to classify costs of an object as either fixed or variable.Choice "a" is incorrect. The engineering method uses such methods as time and motion study to classify costs. It can only be used where there is an observable relationship between the inputs and the outputs.Choice "b" is incorrect. The account analysis method is merely a review of all the accounts by someone knowledgeable of the activities of the firm. It is only as good as the person making the judgments.Choice "d" is incorrect. The high-low method is a simplified approach that uses only the points of highest and lowest activity. The regression method considers every point of activity.