A reaction involving two different reactants can never be
Generally, molecularity of simple reactions is equal to the sum of the number of molecules of reactants involved in the balanced stoichiometric equation. Thus, a reaction involving two different reactants can never be unimolecular.
But a reaction involving two different reactants can be a first order reaction. For example, for the following reaction RCl + H2O → ROH + HCl,
Expected rate law : Rate = k[RCl][H2O] expected order = 1 + 1 = 2. But actual rate law : Rate = k’[RCl] actual order = 1.
Here water is taken in excess, hence its concentration may be taken constant. Here the molecularity of the reaction = 2 and the order of the reaction = 1.
The correct option is A.