Explanation

(d) Magnetic field due to a current carrying wire at a point $P$ which lies at a perpendicular distance $R$ from the wire is given as
$B = \frac{\mu_{0}}{4\pi}\frac{i}{R}\left( \sin{\phi_{1} + \sin\phi_{2}} \right)$
When the linear conductor is of infinite length, then
$\phi_{1} = \phi_{2} = 90{^\circ}$
So, $B = \frac{\mu_{0}}{4\pi R}\ i\ \left\lbrack \sin{90{^\circ}} + \sin{90{^\circ}} \right\rbrack$
$= \frac{\mu_{0}}{4\pi}\frac{2i}{R} = \frac{\mu_{0}}{2\pi}\frac{i}{R}$