A sphere, a cube and a disc all  of the same material, quality and volume are heated to  ${600}^{0}C$ and left in  air. Which of these  will have the lowest rate of cooling?

When a body cools by radiation, according to Stefan’s law,       $\frac{dT}{dt}=\frac{eA\sigma }{mc}(T^4-T_0^4)$
Here,  $m,c,e,T$ and  $T_0$ are same for all bodies; so $\frac{dT}{dt}\propto$   area A
Now,  as for a given mass, area of the sphere is minimum, hence it will have the lowest rate of cooling.