The standard deviation of $\bar{X}$

Question

For a random variable, X, let $\bar{X}$  be the sample average. The sample size is n. the mean and the standard deviation of X are  $\mu$ and $\sigma$ respectively. The standard deviation of $\bar{X}$  is                                                                           

      (a) n$\sigma$                                                                  (b) $\sigma$

      (c) $\frac{\sigma }{n}$                                                                   (d)$\frac{\sigma }{\sqrt{n}}$