The correct option is A 15 cm
Given,
Object distance, u=−30 cm (using sign convention)
Image distance, v=+10 cm (using sign convention - image formed by a convex mirror is always virtual, hence, it is behind the mirror)
We know,
1f=1u+1v, where f is the focal length
1f=1−30+110
Solving this, we get,
f=15 cm
Therefore, the focal length of the given convex mirror is 15 cm.