Assertion: Aluminium is used as a reducing agent in the extraction of chromium from its oxide, $C{r}_2{O}_3$.
Reason: Reduction by Aluminium is known as Gold Schmidt thermite process or Aluminothermic process.

The correct option is B Both assertion and reason are true but the reason is not the correct explanation of assertion
Thermite is a mixture of a metal oxide and another metal powder. Usually, aluminium is used as metal that reduces the metal oxide. Aluminium has a great affinity for oxygen and formation of $A{l}_2{O}_3$ is a highly exothermic reaction. Aluminium acts as a reducing agent when the metal less reactive than Aluminium having a high melting point is to be extracted from its oxide.
$C{r}_2{O}_3$ + $2Al\to 2Cr$ + $A{l}_2{O}_3$
The thermite reaction is also used in the extraction of manganese. This is also an example of a single replacement reaction or redox reaction.