The balanced chemical equation representing the reaction of iron and oxygen is,

The coefficients of each reactant and product represents the number of moles of each reactant or product that is required for the complete reaction to take place.
The mole ratio of iron and iron (III) oxide:
or

Mole ratio of oxygen to iron (III) oxide:
or

So, the mole ratio of iron (Fe) to oxygen gas is: 4 : 3
Therefore, the correct answer is d) 4:3