An argument suggests a new diet plan is effective because 'Dr. Smith, a renowned nutritionist, endorses it.' Which type of additional evidence would be MOST effective in challenging the logical soundness of this argument?

The correct answer to the question "An argument suggests a new diet plan is effective because 'Dr. Smith, a renowned nutritionist, endorses it.' Which type of additional evidence would be MOST effective in challenging the logical soundness of this argument?" is Option C: A controlled scientific study comparing weight loss outcomes for participants on the diet versus a control group..

The other options — A) A testimonial from a satisfied customer who lost weight using the plan., B) A statistic showing the overall popularity of the diet plan in online forums., D) An anecdote about Dr. Smith's personal eating habits, suggesting they don't follow the plan. — are incorrect for this GMAT Exam Prep question. As explained above: While Dr. Smith's endorsement is an appeal to authority, a controlled study provides objective, empirical evidence directly testing the plan's effectiveness, offering the strongest challenge to its c…