Ans: A. Clinical trialAmong the given options, clinical trial is the best study to evaluate effect and outcome.Randomized control trial (Experimental study or Intervention study):Considered as the ideal design to evaluate the effectiveness and the side-effects of new forms of intervention.Coho Study:A study design where one or more samples (called cohos) are followed prospectively and subsequent status evaluations with respect to a disease or outcome are conducted to determine which initial paicipants exposure characteristics (risk factors) are associated with it.