Mean deviation for n observation $x_1,x_2........x_n$ from their mean $\bar{x}$ is given by
$\begin{array}{1 1}(A)\; \sum \limits_{i=1}^n (x_i -\bar {x})\\(B)\;\large\frac{1}{n} \sum \limits_{i=1}^{n} |x_i -\bar {x}|\\(C)\; \sum \limits_{i=1} (x_i -\bar{x})^2\\(D)\;\frac{1}{n} \sum \limits_{i=1}^n (x_i -\bar {x})^2\end{array}$