Answer:
B. A randomized controlled trial
Explanation:
The validity of a study should depend on the research method and how it is designed. A randomized controlled trial is one of the highest valid research methods. Both trial and cohort study is a prospective study, but a trial does intervention while a cohort only does observation.
Randomization will take away bias from sample selection. Control also will take away bias that came from many variables that might influence the result.
