A concave mirror's radius of curvature is $30 \text{cm}$. Its focal length is stated as follows the Cartesian Sign Convention:

 

A concave mirror is one that has the inside surface of a spherical mirror as the reflecting surface. It is also known as a focusing mirror or a converging mirror.

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

$$\begin{gathered} \text{f=}\frac{\text{R}}{2} \\ \text{=}\frac{-30}{2} \\ \text{=-15 cm} \end{gathered}$$

Hence option (D) is correct.