This question is asking for the correct balanced reaction, when solid aluminum reacts with gaseous oxygen to form solid aluminum oxide, according to the law of conservation of mass, which demands us to have the same number of atoms of each element on both sides of the equation.
In such a way, we can start by knowing there is one aluminum atom on the left and two on the right whereas there are two oxygen atoms on the left and three on the right side of the equation.
The aforementioned means we must balance oxygen first by putting a 2 on aluminum oxide and a 3 on oxygen in order to cross multiply and obtain six oxygen atoms on both sides of the equation.
Moreover, the 2 on the aluminum oxide makes four aluminum atoms on the right, which means we must put a 4 on the left-handed aluminum in order to get the correct balance, and therefore, write the following balanced equation:
4Al(s) + 3O₂(g) ⇒ 2Al₂O₃(s)
Hence, the coefficient for Al, as asked, is 4.
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