1. Interpret the statement

The reaction involves two different reactants, so a typical form is:

$A + B \to \text{products}$

We must find what such a reaction can never be.

2. Check each option

Option A: Unimolecular reaction

A unimolecular reaction is an elementary reaction in which only one molecule participates in the rate-determining event, e.g.

$A \to \text{products}$

Since the question says the reaction involves two different reactants, it cannot be unimolecular.

So, A is impossible.

Option B: First order reaction

A reaction involving two reactants can still be first order overall in some cases.

Example:

$A + B \to \text{products}$

with rate law

$\text{Rate} = k[A]^1[B]^0 = k[A]$

or under pseudo-first-order conditions when one reactant is in large excess.

So, such a reaction can be first order.

Option C: Second order reaction

If the rate law is

$\text{Rate} = k[A][B]$

then overall order is

$1+1=2$

So, a reaction involving two reactants can be second order.

Option D: Bimolecular reaction

A bimolecular reaction is an elementary process involving collision of two molecules, such as:

$A + B \to \text{products}$

This exactly matches a reaction involving two different reactants.

So, it can be bimolecular.

3. Conclusion

A reaction involving two different reactants can never be a unimolecular reaction.

$\boxed{\text{A}}$