Choice "D" is correct. When there is a lot of variability within the population, stratification, or separating the population into relatively homogeneous groups, generally results in a reduced sample size and therefore increases efficiency.
Choice "b" is incorrect. A small standard deviation means there is little variability within the population, making it less likely that stratification would be used.
Choice "a" is incorrect. If the population consists of very few items, the auditor will be likely to test each item rather than breaking the population into homogeneous groups.
Choice "c" is incorrect. A population that is unusually homogeneous contains little variability, making it less likely that stratification would be used.