We’re being asked to differentiate a unimolecular and a bimolecular elementary reaction.
Recall that an elementary step represents an actual interaction between the reactant molecules in a step. We characterize elementary steps by their molecularity.
The molecularity of a reaction is the number of reactant particles involved in the step. Here is a table showing the summary of common elementary steps.
What is the difference between a unimolecular and a bimolecular elementary reaction?