The single-sample t-test (to test whether a mean is equal to some specified value) is used when

a. we do not know the value for the population standard deviation (sigma), but we do know the value for the sample standard deviation (s).

b. we know the value for the population standard deviation (sigma).

c. we do know the value of neither the population standard deviation (sigma) nor the sample standard deviation (s).

d. We know the values for both the population standard deviation (sigma) and the sample standard deviation (s).