If $\sin \theta - \cos \theta = \frac{1}{2},$ then the value of $\large\frac{1}{\sin \theta + \cos \theta} =$

Question

$\begin{array}{1 1} (a)\; \large\frac{\sqrt 7}{2} \\ (b)\;\large\frac{2}{\sqrt 7} \\ (c)\; \sqrt{\frac{2}{7}} \\ (d)\; \sqrt{\frac{7}{2}} \end{array}$