Answer: b. The population standard deviation
Explanation:
The one-sample z-test is a test that us carried out to ascertain whether the mean of a population is either greater or less than a specific value given.
The one sample t test is used in comparing the mean of the sample data of the researcher carrying out the research to a known value.
When the population standard deviation is known, the one sample z test is used while when the population standard deviation is not known, one can use the one sample t test.