Optics: JEE Main 2010 Physics question

An initially parallel cylindrical beam travels in a medium of refractive index $\mu \left( I \right) = {\mu _0} + {\mu _2}\,I,$ where ${\mu _0}$ and ${\mu _2}$ are positive constants and $I$ is the intensity of the light beam. The intensity of the beam is decreasing with increasing radius. The initial shape of the wavefront of the beam is