Γ g = dβ / C*dk. What is β in the given equation?

Optoelectronics and Optical Communication 

Γ g = dβ / C*dk. What is β in the given equation?

a) Attenuation constant 
b) Propagation constant 
c) Boltzmann’s constant 
d) Free-space

Answer : - Propagation constant 

Above given equation is an equation of transit time or a group delay(Γg) for a light pulse. This light pulse is propagating along a unit length of a single mode fiber