A hot body will radiate heat most rapidly if its surface is

According to the Stefan-Boltzmann law and Kirchhoff's radiation law, a black surface is a nearly perfect emitter and absorber of thermal radiation (good black body). A rough surface has greater effective surface area for emission compared to a polished one. Therefore, a black and rough surface radiates heat most rapidly. Polished surfaces are poor emitters, while white surfaces reflect more radiation than they emit. This is why radiator fins are often black and rough.