Question

state reason for the following a carbon is not used to reduce the oxide for aluminium to get the metal

Answer 1

Aluminium oxide is very stable and Carbon is less reactive than aluminium, thus it cannot be used to reduce aluminium oxide to aluminium 

Answer 2

By following the reactivity series of metal which gives you the idea that more reactive metal can displace a less reactive metal in chemical reactions
Al2O3 is a stable compound