The correct option is A Corundum
Here, Al2O3 is called corundum.
Corundum is a crystalline form of aluminium oxide Al2O3 that usually contains traces of iron, titanium, vanadium, and chromium. It is a rock-forming mineral that is naturally transparent. Different transition metal impurities present in the structure could impart different colors. The mineral corundum has two principal gem varieties - sapphire and ruby. chromium impurities impart the deep red shade that characterizes rubies. On the other hand, sapphires could display a range of hues depending on the type of transition metal present in the structure.