For spherical mirrors, the correct expression for the relationship between focal length, image's distance, and object's distance is:

 

The following equation expresses the relationship between object distance, image distance, and focal length of a spherical mirror:

$\frac{1}{\text{f}}=\frac{1}{\text{u}}+\frac{1}{\text{v}}$

Where

$\mathrm{f}$ = spherical mirror focal length (distance between the pole and the principal focus of the mirror)
$\mathrm{u}$ = the object's distance (distance between pole and object)
$v$ = the image's distance (distance between pole and image)

Hence option (A) is correct.