Unimolecular reaction involves

• A. Two reactant species.
• B. Three reactant molecules.
• C. Any number of particles.
• D. A single reactant particle.

Answer: (D)

Unimolecular reactions are processes where the transformation happens within a single molecule. The rate depends on just that one species, giving a first-order rate law: rate = k[A]. This means the speed of the reaction is determined by how many molecules of that reactant are present, even though other molecules may be present to provide energy or stabilize intermediates. If the rate-determining step truly required collisions between two or more reactant molecules, the reaction would be bimolecular or termolecular, not unimolecular. So the defining idea is that a single reactant particle is involved in the key step.