Observe the picture given in Fig. carefully. A patch of light is observed at B, when the torch is lighted as shown. Which of the following is kept at position A to get this patch of light? 

In the diagram, we can see how the torch's light is reflecting off the slab and onto the screen. The entire spectrum of light is reflected. The slab would be constructed of a material with the ability to completely reflect the light that strikes it.
We are handed a torch that shines light in the direction of slab A. The light beams are reflected in the direction of screen B. There is no light transmission or absorption happening. A mirror that completely reflects all light rays that strike it is the sole object that possesses this sort of reflectional quality.

The following two rules of light reflection regulate how light is reflected.
1. The incident ray, the reflected ray, and the normal at the point of incidence will always lie in the same plane, according to the first law.
2. The incident ray's angle with respect to the normal at the point of incidence and the reflected ray's angle with respect to the normal at the point of incidence will be equal.
The light hitting a mirror abides by each of these rules.

Hence, the correct answer is option C.