​A study that begins with a group of participants who already have a disease is most likely to be

A) ​a retrospective study.
B) ​a prospective study.
C) ​a correlational study.
D) ​an experimental study.

A study that begins with a group of participants who already have a disease is most likely to be a retrospective study, as it involves looking back at past data and medical records to gather information about the disease and its risk factors. In contrast, a prospective study typically begins with healthy participants and follows them over time to observe potential disease development. Correlational and experimental studies may also involve participants with a disease or a risk factor, but this alone does not determine the study design.