(c) Comparing with $y = a\cos{(\omega t + kx - \phi)},$
We get $k = \frac{2\pi}{\lambda} = 0.02\pi \Rightarrow \lambda = 100\ cm$
Also, it is given that phase difference between particles $\Delta\phi = \frac{\pi}{2}.$ Hence path difference between them
$\Delta = \frac{\lambda}{2\pi} \times \mathrm{\Delta}\phi = \frac{\lambda}{2\pi} \times \frac{\pi}{2} = \frac{\lambda}{4} = \frac{100}{4} = 25\ cm$