What is the type of study done on two groups, one exposed to a potential disease agent and another unexposed?

The correct choice is a cohort study because it specifically involves tracking two groups over time: one group that is exposed to a potential disease agent and another that is not. This design allows researchers to observe the effects of the exposure on the incidence of disease in the exposed group compared to the unexposed group.

Cohort studies are particularly useful for establishing a temporal relationship between exposure and outcome, which is critical for understanding causality. The longitudinal nature of this study enables researchers to gather data at multiple points in time, providing insight into how the potential disease agent may influence the development of disease over a period.

In contrast, a case-control study typically starts with individuals who already have the disease (cases) and compares them to individuals without the disease (controls), looking back to assess exposure. A cross-sectional study evaluates both exposure and outcome simultaneously at a single point in time, making it less suitable for studying causative relationships. Longitudinal studies examine data over time but do not necessarily involve a comparison between exposed and unexposed groups, which is a defining characteristic of cohort studies.