Assertion :Reduction of $F{e}_2{O}_3$ with CO is done below $710^{0}C$.
Reason :$\Delta G$ is negative at this temperature, thus process is spontaneous.

The Ellingham diagram for oxides shows several important features: 

(a) The graph for metal oxide slopes upwards because the free energy change increases with an increase in temperature as shown in the image.

(b)The free energy changes follow a straight line unless the metals vaporize.

(c)When the temperature is raised, a point will be reached where the graph crosses the $\Delta G=0$  line. 

Below this temperature the free energy of formation of the oxide is negative, so the oxide is stable. Above this temperature, the free energy of formation of the oxide is positive, and the oxide becomes unstable, and should decompose into the metal and dioxygen. 

(d) Any metal will reduce the oxide of other metals which lie above it in the Ellingham diagram because the free energy will become more negative by an amount equal to the difference between the two graphs at that particular temperature.

(e) $Al$ metal can reduce the oxide of metal placed above it in the diagram. Due to more negative free energy, $Al$ can reduce $Zn$ but not $Mg$.

As $\Delta G=0$ is negative below $710^{0}C$, the process is spontaneous and $F{e}_2{O}_3$can be reduced by $CO$ below this.

Hence, both Assertion and Reason are correct and Reason is the correct explanation for Assertion. 

So, option A is correct.