An object is placed at a distance \(d_o\) in front of a spherical mirror with focal length \(f\). According to the standard sign convention, \(f > 0\) for a concave mirror and \(f < 0\) for a convex mirror. Which of the following expressions correctly gives the lateral magnification \(M\) of the image purely as a function of \(f\) and \(d_o\)?