This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
Causal association can be best established in - 
Select and explain. 
A. RCT, B. Coho study, C. Case control study, D. Ecological study
A: RCT
Answer: A: RCT 
Explanation: Ans.'a'i.e.,C (Ref a single study unit, double blind RCT is the best.However, overall meta-analysis is a better study since it combines the data from multiple RCT and also from other types of study."Randomized controlled trials (RCT) provide the stongest, most relevant evidence to inform practice. Some evidence hierarchies place systematic review and meta-analysis above RCTs since these often combine data from multiple RCTs, and possibly from other study type as well" -- Epitemiology at glanceSo, systematic review and meta-analysis of RCTs are best epidemiological studies