A short linear object of length b lies along the axis of a concave mirror of focal length f at a distance u from pole of mirror , The size of image is approximate equal to :

Question

$(a)\;\frac{bf}{u-f} \\ (b)\;\frac{b^2 f}{u-f} \\ (c)\;\frac{bf}{(u-f)^2} \\ (d)\;b\bigg ( \frac{f}{u-f}\bigg)^2 $