Choice "D" is correct. As long as the population is large enough, population size has virtually no impact on sample size.
Choice "c" is incorrect. The tolerable rate of deviation does affect sample size. As the auditor is willing to accept a greater deviation rate, a smaller sample size can be used.
Choice "a" is incorrect. The degree of assurance required does affect sample size. As the auditor desires greater assurance from the sample, a larger sample size should be used.
Choice "b" is incorrect. The planned assessed level of control risk is related to the expected deviation rate (a low expected deviation rate corresponds to a low planned assessed level of control risk). Therefore, changes to this assessment do affect sample size.