a b c d

c

Since the image is real, it must be inverted and magnification ratio should be negative.   u =-x M = -n = $\frac{-v}{-x}$ $\Rightarrow v = -n.x $ $\frac{1}{-f} = \frac{1}{v} + \frac{1}{u} = \frac{1}{-nx} + \frac{1}{-x}$ $ x = \frac{(n+1)f}{n}$