At a dui checkpoint, some cars are stopped at random; many others are not stopped. using the terminology of psychological research, the cars that are stopped may be considered a of all the cars that pass by the checkpoint.
a)sample
b)subset
c)cohort
d)population

a) sample.

Explanation:

When we do research we usually work with populations or samples:

In this example, at a DUI checkpoint, some cars are stopped at random. We can see that ALL the cars that pass by the checkpoint would be considered the population (since they are all the elements of the set) and the ones that are stopped are SOME elements that belong to the original set, therefore, they can be considered a sample of all the cars that pass by the checkpoint.

debating and finalizing the detail of the policy