b
In the medium, the refractive index will decrease from the axis towards the periphery of the beam.

Therefore, the beam will move as one move from the axis to the periphery and hence the beam will converge.

Initially the parallel beam is cylindrical. Therefore, the wavefront will be planar.

The speed of light \((c)\) in a medium of refractive index \((\mu)\) is given by

\(\mu=\frac{c_{0}}{c},\) where \(c_{0}\) is the speed of light in vacuum

\(\therefore c=\frac{c_{0}}{\mu}=\frac{c_{0}}{\mu_{0}+\mu_{2}(I)}\)

As \(I\) is decreasing with increasing radius, it is maximum on the axis of the beam. Therefore, \(c\) is minimum on the axis of the beam.