A reaction involving two different reactants can never be

Molecularity is defined as the number of reacting species (atoms, ions or molecules) that must collide simultaneously to bring about a chemical change in an elementary step.

If a reaction step involves two different reactants, say $$A$$ and $$B$$, then at the very instant of the elementary event one molecule of $$A$$ and one molecule of $$B$$ must collide. Hence the minimum number of molecules participating is two.

Therefore, such an elementary step must be bimolecular. It can never be unimolecular because that would require only one molecule to undergo change, which is impossible when two different reactants are present.

• Order of the overall reaction can still be first (pseudo-first when one reactant is in large excess), or second (normal case $$\text{rate}=k[A][B]$$).
• It is always bimolecular in terms of molecularity.

Hence, the only impossible description is “unimolecular reaction.”

Option A which is: Unimolecular reaction