The limiting reactant in a chemical equation is the reactant that is completely consumed when the chemical reaction takes place. As a result the reactant determines for how long the reaction takes place.
The reaction given is that between aluminum and oxygen to yield Al2O3. The equation of this chemical reaction is:
4Al + 3O2 --> 2Al2O3
3.17 g of Al and 2.55 g of O2 are available for the reaction to take place. The molar mass of Al is 27 and that of O2 is 32.
3.17 g of Al is equivalent to 0.11 moles and 2.55 g of O2 is equivalent to 0.079 moles
As 4 moles of Al react with 3 moles of oxygen in the reaction, there should be 3/4 times as much oxygen as aluminum. But the number of moles of the reactants available to react shows that a lesser amount of oxygen is available.
In this case, oxygen is limiting reagent and the reaction stops when all the oxygen is consumed.