A unit vector coplanar with the vectors $\hat i-\hat j\:\;and\:\:\hat i+2\hat j$ and is $\perp$ to the first vector, $\hat i-\hat j$ is ?
$\begin{array}{1 1} (a)\:\large\frac{1}{\sqrt 2}(\hat i+\hat j)\:\:\\ (b)\:\large\frac{1}{\sqrt 5} (2\hat i+\hat j)\\(c)\:\large\frac{1}{\sqrt 2}(\hat i+\hat k)\\(d)\:None\:of\:these. \end{array}$