1. Key idea: shape of emergent wavefront depends on optical path through the glass

   When a plane wavefront enters a glass slab of non-uniform thickness, different parts of the wave travel through different thicknesses of glass.

   Since refractive index of glass is $\mu>1$, light travels more slowly in glass, so a larger glass thickness produces a larger optical delay.

2. Optical path difference across the beam

   If a ray passes through glass thickness $t$, then compared to the same distance in air, the additional optical path is $\Delta = (\mu-1)t.$

   Hence:

   • where the glass is thicker, the wavefront is more delayed,
   • where the glass is thinner, the wavefront is less delayed.

3. Nature of the given glass piece

   From the shown cross section, the thickness is not uniform; it is greater in the middle than near the edges (schematic lens-like shape).

   Therefore:

   • central portion of the wavefront suffers more delay,
   • outer portions suffer less delay.

4. Resulting emergent wavefront

   Since the center lags behind and the edges advance relative to it, the emergent wavefront becomes curved inward at the center.

   That corresponds to a converging-type wavefront after emergence.

5. Matching with options

   The option showing the center delayed relative to the edges is Option A.

Final Answer

$\boxed{\text{A}}$