The correct answer is: Group velocity.
Group velocity is the velocity at which information propagates in a dispersive medium. It is the velocity at which a pulse of light travels through an optical fiber.
Phase velocity is the velocity at which a wave travels through a medium. It is the velocity at which a point on a wave travels.
Normalized velocity is a dimensionless quantity that is used to compare the phase velocity and group velocity of a wave. It is defined as the ratio of the group velocity to the phase velocity.
Average velocity is the average of the phase velocity and the group velocity. It is the velocity at which a wave would travel if it were not dispersive.
In the study of transmission characteristics of optical fiber, group velocity is the most important velocity because it determines the speed at which information propagates through the fiber.