A study is designed where investigators use old hospital records to identify exposure and then follow patients’ outcomes from the past. This is best described as:

Correct option is C

In retrospective cohort, both exposure and outcome have already occurred, and data is collected from past records.
Explanation of options: (a) ✘ Cross-sectional = exposure + outcome measured at single point in present. (b) ✘ Prospective cohort = follow-up planned forward in time. (c) ✔ Correct; retrospective cohort = past records of exposure + outcomes. (d) ✘ Case–control = starts with disease status, not exposure group.