Volunteers in a study are assigned randomly to groups. Some of the volunteers receive an herbal supplement that is reputed to control nausea, and some of the volunteers are assigned to a control group where a placebo is administered. This is an example of a(n):

The correct answer is D: RCT (Randomized Controlled Trial). In this study design, volunteers are randomly assigned to groups to receive either the herbal supplement or a placebo, allowing for comparison of the effects. This random assignment helps minimize bias and ensures that any observed differences between the groups are likely due to the treatment. A longitudinal study (A) follows subjects over an extended period, which is not described here. An appraisal tool (B) is used to evaluate research quality, not a study design. Meta-analysis (C) involves combining results from multiple studies, not assigning treatments.