Corundum is composed of about 85 percent of-

Correct option is A

Corundum is a crystalline form of aluminium oxide (Al₂O₃) and is typically composed of about 85% aluminium oxide. It is a naturally occurring mineral and is one of the hardest materials, second only to diamond. Corundum is widely used as an abrasive due to its hardness and is also the base material for precious gemstones such as sapphires and rubies.