Explanation

(a) $B_{1} = B_{3} = B_{5} = 0$
$B_{2} = \frac{\mu_{0}}{4\pi}.\frac{\theta i}{3r} \otimes ,\ B_{4} = \frac{\mu_{0}}{4\pi}.\frac{\theta i}{2r} \odot$
and $B_{6} = \frac{\mu_{0}}{4\pi}.\frac{\theta i}{r} \otimes$
$\therefore$ Net magnetic field at $O$,
$B_{net} = B_{2} - B_{4} + B_{6} = \frac{\mu_{0}}{4\pi}.\frac{\theta i}{r}\left( \frac{1}{3} - \frac{1}{2} + 1 \right) = \frac{5\mu_{0}\theta i}{24\pi r}$