(A) (i) The point of the principle axis from where all the incident rays appear to diverge from behind the mirror, is called principle focus of the diverging mirror.
(ii) The distance between the pole of the mirror and its principal focus is called focal length.(B) Object height, $\mathrm{O}=+10\mathrm{cm}$
Focal length, $\mathit{f}=+15\mathrm{cm}$
Object distance, $\mathit{u}=-25\mathrm{cm}$
Image distance, $\mathit{v}=$ ?
Image height, $\mathrm{I}=$ ?
Using lens formula, $\frac{1}{\mathit{f}}=\frac{1}{\mathit{v}}-\frac{1}{\mathit{u}}$
$\frac{1}{15}=\frac{1}{\mathit{v}}-\frac{1}{\left(-25\right)}$
Or,
$\frac{1}{\mathit{v}}=\frac{1}{15}-\frac{1}{25}$
$\frac{1}{\mathit{v}}=\frac{2}{75}$
$\mathit{v}=\frac{75}{2}$
Also, $\frac{-\mathit{v}}{\mathit{u}}=\frac{\mathrm{I}}{\mathrm{O}}$
Hence, $\frac{-\frac{75}{2}}{-25}=\frac{\mathit{I}}{10}$
$\mathit{I}=\left(-1.5\right)\times 10=15\mathrm{cm}$
Height of image is $15\mathrm{cm}$, inverted.