1. Interpret the geometry

• Radius of hemispherical glass body: $R = 10\,\text{cm}$
• Refractive index of glass: $\mu = 1.5$
• The curved surface is silvered, so it acts as a spherical mirror.
• The air bubble lies on the axis, $6\,\text{cm}$ below the flat surface.

Since the center of the hemisphere is at the flat face center, the pole of the curved mirror is $10\,\text{cm}$ below the flat surface.

So the bubble is at a distance from the curved mirror pole: $u = 10 - 6 = 4\,\text{cm}$

2. Image formed by the silvered curved surface (mirror action inside glass)

The silvered curved surface behaves like a concave mirror for rays inside the glass.

For a spherical mirror, $f = \frac{R}{2} = \frac{10}{2} = 5\,\text{cm}$

Using mirror formula, $\frac{1}{f} = \frac{1}{u} + \frac{1}{v}$

Substitute $u=4\,\text{cm}$ and $f=5\,\text{cm}$: $\frac{1}{5} = \frac{1}{4} + \frac{1}{v}$ $\frac{1}{v} = \frac{1}{5} - \frac{1}{4} = \frac{4-5}{20} = -\frac{1}{20}$ $v = -20\,\text{cm}$

The negative sign means the image lies behind the mirror (with this sign convention), i.e. on the other side of the pole by $20\,\text{cm}$.

Since the mirror pole is $10\,\text{cm}$ below the flat surface, the image position relative to the flat surface is: $10 + 20 = 30\,\text{cm}$

This is the image formed by the mirror inside glass.

3. Seen through the plane flat surface

Now this image at $30\,\text{cm}$ below the flat surface is viewed from air through the plane glass surface.

For refraction at a plane surface, $\frac{\mu_2}{v} - \frac{\mu_1}{u} = 0$ which gives apparent depth relation: $\text{apparent depth} = \frac{\text{real depth}}{\mu}$

Here real depth in glass $= 30\,\text{cm}$, and $\mu = 1.5$.

Hence apparent depth from air is $d_{\text{app}} = \frac{30}{1.5} = 20\,\text{cm}$

4. Final answer

Thus, the image of the air bubble made by the mirror is seen at $\boxed{20\,\text{cm below the flat surface}}$

So the correct option is C.