Answer:
The correct answer is option A. "An experiment using a completely randomized design".
Step-by-step explanation:
One of the main criteria that a clinical study must follow is that the experiment should be designed using a completely randomized approach. A completely randomized design is independent of the bias of people that are performing the experiments and the people that is taking the medicine. This is the case in this example, where the 500 participants were randomly assigned to groups that were of equal size.