Causal association can be best established in -

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