In which type of research are subjects randomly assigned to different groups?

The correct choice refers to experimental research, which is characterized by the active manipulation of variables and the random assignment of subjects to different groups. This method allows researchers to establish cause-and-effect relationships by controlling for external factors that might influence the results.

In experimental research, random assignment helps ensure that each group is similar at the outset, which minimizes the influence of confounding variables. This approach enhances the validity of the findings, as it allows researchers to determine the specific effects of an intervention or treatment on the subjects assigned to each group.

The other types of research listed do not typically involve random assignment of subjects. Case study research focuses on in-depth analysis of a single subject or small group, often without any control over variables. Qualitative research aims to understand complex phenomena through non-numeric data and often involves smaller, purposefully selected samples rather than random assignment. Retrospective research examines existing data from past events or outcomes, which does not involve any experimental manipulation or group assignment. Thus, the random assignment of subjects is a hallmark feature unique to experimental research.