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.