A monochromatic wave propagates along a waveguide in z direction. These points of constant phase travel in constant phase travel at a phase velocity V p is given by?

Optoelectronics and Optical Communication 

A monochromatic wave propagates along a waveguide in z direction. These points of constant phase travel in constant phase travel at a phase velocity V p is given by?

a) V p =ω/β 
b) V p =ω/c 
c) V p =C/N 
d) V p =mass/acceleration

Answer : - V p =ω/β