A. The reagents are in molar ratios shown in the reaction ecuation. In this case the reagents are in stoichiometric proportions and when the reaction is complete only will be products in the reaction medium.
B. One of the reactants is in default (less than the stoichiometric molar ratio). Is the limiting reagent. When this reagent is consumed the reaction can not continue.
The stoichiometric ratio is given by the coefficients of the balanced chemical equation. In reaction A2 + B2 → 2AB, the two reactants are combined in te same proportions (1 mol + 1 mol). The stoichiometric mixture must have the same number of moles of each reactant. If one of them is to a lesser extent, will be the limiting reagent.
In a reaction 2A2 + B2 → A2B, the two reactants are combined in proportions 2 mol of A2 + 1 mol of B2. The stoichiometric mixture must have twice as many moles of reactant A. If one of them is less than the stoichiometric ratio, will the limiting reagent.