mole ratio is the ratio between the amount in moles of any two compounds involved in a chemical reaction
3
the mole of Fe3O4 is 1 mole while that of Fe is 3 moles from the reacting equation below
Fe + 4 H2O ----> Fe3O4 + 4 H2
therefore the mole ratio of Fe3O4 to Fe is 1:3(answer A)