Answer:
Conducting An Experiment
Step-by-step explanation:
The most appropriate research method will definitely be "conducting an experiment".
In as much as the researcher wants to find out the effect of watching a documentary about climate change on a subject’s attitudes toward "green" practices, he can only achieve that by conducting an experiment.
The experiment carried out will provide an evidence on which he can draw conclusions for or against the hypothesis. The experiment conducted will provide a basis for final assessment, judgment and conclusion.