We can find the standard deviation of $$3,7,9,13 \& 18$$ about the mean $$(=\overline{\mathrm{X)$$ i.e. $$\frac{3+7+9+13+18}{5}=\frac{50}{5}=10 ($$$\frac{ Arithmetic Mean}{Average} =\frac{ Sum of terms }{ Number of terms })$$ so we get

$$\begin{tabular}{|l|l|l|} \hline $x_i$ & $x_i-$ Mean $=x_i-10$ & $\left(x_i-\text { median }\right)^2$ \\ \hline 3 & -7 & 49 \\ \hline 7 & -3 & 9 \\ \hline 9 & -1 & 1 \\ \hline 13 & 3 & 9 \\ \hline 18 & 8 & 64 \\ \hline & {$\sum\left(x_i-\bar{x}\right)^2=132$} \\ \hline \end{tabular}$$

So, the standard deviation is $$\sqrt{\frac{\sum(x_i-\bar{X})^2}{n}$$=\sqrt{\frac{132}{5}}=\sqrt{26.2} \approx 5.12$$

Hence the answer is (D).