Answer:
answer : a. 4
Step-by-step explanation:
if u look at the reactants, the coefficient of the Al is 4, while the product the coefficient is 2. By the way, if u look properly u can see that at the product side, the Al is written as Al2, which means that, the coefficient 2 is need to be multiply by 2 from the Al2.
Reactant = 4Al
Product = 2Al2, therefore the 2 at the Al2, should be multiply be the coefficient to make it 4.