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I suppose A) Sun ,  as the sun is farthest.    so  1/u = 0.    v = f.

     The objects  Sun and a well illuminated building are  good sources of light.  Because they are distant, the rays from them on to the small concave mirror are parallel.  Thus, the rays converge at the focal point.   You need a very small piece of paper or screen to capture the image.  In fact,  the image will be a small bright spot.  The Sun may be a better source in the day.   But to be careful the screen (or the paper) may burn due to the concentration of heat at the focus.

     The class room window and the distant tree are not light sources themselves.  So they may not reflect a lot of light.  As the image is very small at the focal point, we cannot identify the image of the tree or window.

1 5 1
the above is the good explanation. what is possible. please understand the concept. that is more important than the exact answer
this is a question of mcq u have to choose only one answer
The sun is the best choice as it is a light source and its distance from the mirror is nearly infinity.