Areinforcer is any reward that naturally increases the frequency of a target behavior.

I believe the answer is: Primary

For example, let's say take a look at normal employees at a company.

For most of them, the main reason they spend their time to do the job correctly is to obtain money/salary even though they have to spend their time being scolded by unlikable boss.

In the scenario above, we can say that money is the primary reinforcer for the target behavior