When λ is the optical wavelength in vacuum, k is given by k=2Π/λ. What does k stand for in the above equation?

Optoelectronics and Optical Communication 

When λ is the optical wavelength in vacuum, k is given by k=2Π/λ. What does k stand for in the above equation?

a) Phase propagation constant 
b) Dielectric constant 
c) Boltzmann’s constant 
d) Free-space constant 

Answer : - Phase propagation constant