D. The subscript 2 indicates that two atoms of aluminum are present in the substance.
Step-by-step explanation:
Actually, the formula of alumina is Al₂O₃
The subscript 2 tells us that there are 2 Al (Aluminum) atoms, and the subscript 3 says that there are 3 oxygen atoms, so D is the accurate statement.
Choice A is wrong as 3 is not the coefficient, and it is a subscript.
Choice B is wrong since the subscript 2 meant for the aluminium atoms, and not the oxygen atoms.
Finally, choice C is also wrong since the symbol Al refers to the metal aluminium, and not the oxygen.