Explanation

(c) The relation between velocity, frequency and wavelength is
$v = n\lambda\ \ \ \ \ or\ \ \ \lambda = \frac{v}{n}$
$Given,\ v = 360ms^{- 1},\ \ n = 500\ Hz\ $
$\therefore\ \ \ \ \lambda = \frac{360}{500} = 0.72m$
$Path\ difference\ = \frac{\lambda}{2\pi} \times phase\ defference$
$I,\ e.,\ \ \ \ \ \ \ \ \mathrm{\Delta}x = \frac{\lambda}{2\pi} \times \phi$
$or\ \ \ \ \ \ \ \mathrm{\Delta}x = \frac{0.72}{2\pi} \times \frac{\pi}{3}\ \ \ \ \ \left( \therefore\mathrm{\Delta}\phi = 60{^\circ} = \frac{\pi}{3} \right)$
=0.12m=12 cm