b
First particle is comming towards mean position
$\therefore \phi_{1}=\pi-\frac{\pi}{6}$
second particle is comming towards mean position but in negative side
$\therefore \phi_{2}=2 \pi-\frac{\pi}{6}$
$\therefore \phi_{2}-\phi_{1}=\left(2 \pi-\frac{\pi}{6}\right)-\left(\pi-\frac{\pi}{6}\right)=\pi$