3Fe3O4 + 4Al → 9Fe + 4Al2O3:

Count each type of atom on both sides. The left side has 9 Fe, 12 O, and 4 Al atoms; the right side has 9 Fe, 12 O, and 8 Al atoms. Since the aluminum counts differ, the equation is unbalanced, so the correct choice is B) unbalanced.

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