(B) – 20 cm
Using mirror formula: $u = -30$ cm, $v = -60$ cm (real image, in front of mirror).
$$\frac{1}{f} = \frac{1}{v} + \frac{1}{u} = \frac{1}{-60} + \frac{1}{-30} = \frac{-1-2}{60} = \frac{-3}{60} = \frac{1}{-20}$$
So, $f = -20$ cm.
Apply the mirror formula with correct signs (New Cartesian Sign Convention): object and real image both in front of the mirror, so both u and v are negative. A concave mirror always has a negative focal length. Students commonly make sign errors — always assign signs before substituting.