Correct Answer  Option 3 : (n+1)f/n
CONCEPT:

Concave mirror: If the inner surface of the spherical mirror is the reflecting surface, then it is called a concave mirror. It is also called a focusing mirror/converging mirror.
 The size of the image produced by these mirrors can be larger or smaller than the object, depending upon the distance of the object from the mirror.
 The concave mirror can form both real as well as virtual images of any object.

Mirror formula: The expression which shows the relation between object distance (u), image distance (v), and focal length (f) is called mirror formula.
\(\frac{1}{v} + \frac{1}{u} = \frac{1}{f}\)
Linear magnification (m):
 It is defined as the ratio of the height of the image (hi) to the height of the object (ho).
\(m = \frac{{{h_i}}}{{{h_o}}}\)
 The ratio of image distance to the object distance is called linear magnification.
\(m = \frac{{image\;distance\;\left( v \right)}}{{object\;distance\;\left( u \right)}} =  \frac{v}{u}\)
 A positive value of magnification means virtual an erect image.
 A negative value of magnification means a real and inverted image.
CALCULATION:
Given f = focal length and m = n (negative sign shows that the image will be inverted to the object)
Since,
\(⇒m= {{h'} \over h}=\frac{v}{u}=n\)
⇒ v = nu (1)
The mirror formula is given as,
\(\Rightarrow \frac{1}{f}=\frac{1}{v}+\frac{1}{u}\) (2)
By equation 1 and equation 2,
\(\Rightarrow \frac{1}{f}=\frac{1}{nu}+\frac{1}{u}\)
\(\Rightarrow \frac{1}{f}=\frac{n+1}{nu}\)
\(\Rightarrow u=\frac{(n+1)}{n}f\)
 Hence, option 3 is correct.