Hot water is poured simultaneously in four metallic tumblers painted outside with different points. After some time the water will be found to have cooled most in the tumbler painted.

Black  surfaces are good emitters of heat ( They are also good absorbers of heat). If the black surface is rough too, the emitting will be better. So (b) is the correct answer. Shining surfaces reflect back radiation. So if the black surface is also shining the effectiveness of emitting heat is lessened.