In Which of the Following Situations Is the Calculation of the Z-Statistic

In which of the following situations is the calculation of the z-statistic NOT appropriate for making inferences about a single mean?

A) The variable of interest is normally distributed in the population, $\sigma$ is known, and n (number of observations) is large.
B) The variable of interest is normally distributed in the population, $\sigma$ is known, and n is small.
C) The variable of interest is normally distributed in the population, $\sigma$ is unknown, and n is small.
D) The variable of interest is not normally distributed in the population, $\sigma$ is known, and n is large.
E) Calculation of the z-statistic is appropriate in each of these situations.