A spherical mirror is one with a reflecting surface that is part of a hollow glass sphere.

Spherical mirrors are classified into two types: convex mirrors and concave mirrors.

The link between focal length and aperture ($\mathrm{f}$)as well as the radius of curvature ($\mathrm{R}$) -

$$\begin{gathered} \text{f=}\frac{\text{R}}{2} \\ \text{R=2f} \end{gathered}$$

Hence option (B) is correct.