A concave mirror of focal length f produces a real image at a distance v from the pole, when an object is kept at the distance a from the pole. Here, focal length of the given minor is:

Correct option is C

The correct answer is: (C). (uv) / (u + v)

The formula relating object distance (u), image distance (v), and focal length (f) for a concave mirror is given by the mirror equation:

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

Rearranging this equation to solve for the focal length (f):

                                           $$f = \frac{uv}{u + v}$$​

This shows that the focal length f of the concave mirror is given by:

                                         $$f = \frac{uv}{u + v}$$​

So, the correct answer is indeed (C). (uv) / (u + v).