If the magnitude of moment about the point $\overrightarrow{j}+\overrightarrow{k}$ of a force $\overrightarrow{i}+\overrightarrow{aj}-\overrightarrow{k}$ acting through the point $\overrightarrow{i}+\overrightarrow{j} $ is $\sqrt{8} $ then the value of $a$ is

Question

\[\begin{array}{1 1}(1)1&(2)2\\(3)3&(4)4\end{array}\]