b
Given, radius of herrispherical glass \(\mathrm{R}=10\, \mathrm{cm}\)

\(\therefore\) Focal length \(\mathrm{f}=\frac{10}{2}=-5 \,\mathrm{cm}\)

\(\mathrm{u}=(10-6)=-4\, \mathrm{cm}\)

By using mirror formula,

\(\frac{1}{v}+\frac{1}{u}=\frac{1}{f} \Rightarrow \frac{1}{v}+\frac{1}{-4}=\frac{1}{-5}\)

\(\Rightarrow v=20 \,\mathrm{cm}\)

Apparen the height,

\(\mathrm{h}_{\mathrm{a}}=\mathrm{h}_{\mathrm{r}} \frac{\mu_{1}}{\mu_{2}}=30 \times \frac{1}{15}=20\, \mathrm{cm}\) below flat surface.