Choice "B" is correct. Ratio estimation is most effective if there is a correlation between book values and audit amounts.
Choice "a" is incorrect. A relatively small number of differences do not improve the effectiveness of ratio estimation sampling relative to other techniques.
Choice "d" is incorrect. Whether populations are stated in terms of quantities or book values is irrelevant in determining the effectiveness of ratio estimation.
Choice "c" is incorrect. Large overstatement and understatement errors do not give the ratio estimation sampling technique an advantage in effectiveness relative to other sampling techniques.