A straight wire of length $\ell$ carries a current $i$. The magnitude of magnetic field at the point $P$ is

Question

 

$\begin {array} {1 1} (a)\;\large\frac{\sqrt 2 \mu_0i}{\pi l} & \quad (b)\;\large\frac{\sqrt 2 \mu_0i}{2\pi l} \\ (c)\;\large\frac{\sqrt 2 \mu_0i}{4\pi l} & \quad (d)\;\large\frac{\sqrt 2 \mu_0i}{8\pi l} \end {array}$