Loss of heat temperature on cooling temperature increase depend on material of object surface area exposed to surrounding and temperature difference between body and surrounding.
Let us consider the diagram where all the three objects are heated to sametemperature $T$. As we know that density
$$
\rho=\frac{\text { mass }}{\text { volume }}
$$
where $\rho$ is same for all the three objects hence, volume will also be same.


As thickness of the plate is least so, surface area of the plate is maximum.
We know that, according to Stefan's law of heat loss
$H \propto A T^4$
where, $A$ is surface area of object and $T$ is temperature.
So, $H_{\text {sphere }}: H_{\text {cube }}: H_{\text {plate }}$
$=A_{\text {sphere }}: A_{\text {cube }}: A_{\text {plate }}$
So area of circular plate is maximum.
For sphere, as the sphere is having minimum surface area.
Hence, the sphere cools slowest and circular plate will cool faster.