Which of the Following Assumptions Must Be True in Order $\bar { x } \pm z _ {\alpha / 2 } \sigma / \sqrt { n }$

Short Answer

Which of the following assumptions must be true in order to use the formula $\bar { x } \pm z _ {\alpha / 2 } \sigma / \sqrt { n }$ to find a confidence interval estimate of the population mean? $\begin{array}{|l|l|}\hline A&\text {The population variance is known. }\\\hline B&\text { The population mean is known.}\\\hline C&\text { The population is normally distributed.}\\\hline D&\text {The confidence level is greater than \( 90 \% \). }\\\hline \end{array}$