(c) r1​.Let us consider two coaxial cylindrical surfaces at distances r and r′ from the axis. Let areas dA and dA subtend the solid angle dω at the central axis. The height of the area element will be same, i.e. equal to dy. Let the breath of dA be dx and that of dA′ be dx′ .Now from the arc s ,dx=rdθdx′=r′dθ Now,dA=dxdy=rdθdydA′=dx′dy=r′dθdydA′dA​=r′r​⇒rdA​=r′dA′​=dω The luminous flux going through the solid angle dω will be:dF=Idω Now,dF=IrdA​ If the surfaces are inclined at an angle α ,dF=IrdAcosα​ Now, illuminance is defined asE=dAdF​=IrdAcosα​⇒E∝r1​