The correct answer is A) true.
It is true that a cohort is a group of people that have shared a particular time or time-span together.
This concept of "cohort" has been applied by researchers to define a population and study some factors. For instance, a generation in College. The group of people that started college as Freshmen until they graduate as seniors. They lived four years together, in similar facilities, classrooms, and activities. So this helps researchers to get some interest conclusions on how people live and behave.