Answer:
c. 3
Explanation:
The unbalanced reaction expression is given as:
     Fe  +  Oâ‚‚  →  Feâ‚‚O₃ Â
To balance the expression gives the coefficient of oxygen.
Assign the coefficients a,b, c and use a mathematical approach to solve this problem:
       aFe  +  bO₂  →  cFe₂O₃
Conserving Fe: a  = 2c
          O:  2b  = 3c
Now let c = 1, a  = 2, b  = [tex]\frac{3}{2}[/tex]
 Multiply through by 2,
       a = 4, b = 3 and c  = 2
Â
        4Fe  +  3Oâ‚‚  →  2Feâ‚‚O₃ Â
The coefficient of Oâ‚‚ is 3