Which study design provides the highest level of evidence among those listed?

Correct answer: C. Randomised controlled trial

• A. A case series simply describes a group of patients with no comparison group at all — near the bottom of the hierarchy.
• B. A cohort study follows exposed and unexposed groups forward and yields relative risk and incidence, but without randomisation confounding remains.
• C. Correct. Randomisation distributes known and unknown confounders evenly between groups, so the RCT gives the strongest causal inference of the designs listed.
• D. Case-control studies work backward from outcome to exposure. They are efficient for rare diseases but give odds ratios and are prone to recall bias.

Note: When a systematic review or meta-analysis appears among the options, it outranks the individual RCT. Here it was not offered.