Which of the following best describes a cohort study?

A cohort study is best described as a longitudinal study that observes a group over time to assess health outcomes. This design involves selecting a group of individuals who share a common characteristic, often related to a specific exposure, and monitoring them over a period to observe how that exposure affects their health. This method is particularly effective for identifying associations between risk factors and health outcomes, as it allows researchers to track changes and developments within the cohort over time.

In contrast, the other choices define different study designs. A study that separates individuals based on their health conditions focuses more on categorization rather than long-term observation. A cross-sectional analysis looks at a population at a single point in time, providing a snapshot of current health status rather than tracking changes over time. Finally, a study that concentrates on a single individual or small group lacks the broader perspective necessary to make generalizations about larger populations, which is a hallmark of cohort studies. Thus, the defining characteristic of a cohort study lies in its longitudinal nature and focus on group health outcomes.