Final answer:
A study following a well-defined population over time, collecting data periodically, is known as a Cohort study. It is prospective in nature, providing accurate and real-time data unlike retrospective case-control studies.
Step-by-step explanation:
A study in which a group comes from a well-defined population and is followed over a long period of time is a B. Cohort study. A cohort study is an observational study that identifies subjects at the start of the study (the cohort) and follows them prospectively into the future. This allows researchers to collect data at various points in time to observe progression and changes within the group. Unlike case-control studies, which are retrospective and rely on subjects' recall of past behaviors, cohort studies provide more accurate, real-time data as the events unfold. However, cohort studies are more costly and challenging to conduct compared to other observational studies like cross-sectional or case-control studies.