(a), (b), (c), (d) All the options are correct. a) Moving the source to the middle will illuminate the surface properly because illuminance depends upon the distance from the source.b) Rotating the source will also have an effect because illuminance depends upon the angle made by the normal on the surface.c) Bringing mirrors to the proper position will increase illuminance at that particular portion of the wall by gathering light and focussing them at one point.d) Our eyes sense some colours as bright and some colours as dull, selecting the colours near yellow will make the wall appear brighter.