Aluminium oxide is not reduced by chemical reaction since:
A. Reducing agent contaminates
B. The process pollutes the environment
C. Aluminium oxide is highly stable
D. Aluminium oxide is reactive

Hint: Aluminium oxide or aluminum oxide is the name of a chemical compound containing aluminium and oxygen.
Aluminium oxide has the chemical formula ${\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}$ .
The method of reduction of metal oxides depends upon the reactivity of the metal.

Complete step by step answer:
Aluminium oxide is one of the most commonly occurring oxide of aluminium. It is also known as alumina. The most commonly naturally occurring crystalline form of aluminium oxide is the mineral corundum.
Aluminium oxide is a highly stable oxide of aluminium. Thus, it is chemically inert.
Metal oxides can be reduced to the corresponding metal by using a suitable reducing agent. But the choice of the reducing agent depends upon the reactivity of the metal.
If the metal to be produced is a very reactive metal like alkali metals, alkaline earth metals and aluminium, then the reduction of the oxide can be done by only the electrolytic method. The oxides of less reactive metals like zinc, iron etc. are reduced by a number of reducing agents like carbon or coke or carbon monoxide.
This is why aluminium is extracted from its ore bauxite by electrolytic reduction. The bauxite ore is first converted into alumina or ${\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}$ which is then converted into aluminium by the electrolysis of mixture of alumina and cryolite via the Hall and Heroult process.

So, the correct option is C.

Note:
Aluminium oxide is used as a filler for plastics as it is chemically inert and white. It is a common ingredient in sunscreen and is also present sometimes in cosmetics like nail polish, lipstick and blush.
Many different kinds of glass have aluminium oxide as a common constituent. For example, aluminosilicate glass contains 5 to 10 percent alumina or aluminium oxide.
Aluminium oxide also acts as an industrial catalyst. It is used as a catalyst in the Claus process which is the conversion of hydrogen sulfide waste gases into elemental sulfur. It is also used in the dehydration of alcohols to alkenes.