A major drawback of the repeated measures design is:

The correct choice highlights a significant limitation of the repeated measures design, which is the occurrence of order effects. In a repeated measures design, the same participants are used across all conditions of the experiment. This means that their performance in one condition can be influenced by their experience in previous conditions. For instance, participants might become fatigued or practice the task when they complete it multiple times, leading to changes in their responses that are not related to the independent variable being tested. This can undermine the validity of the results, as any observed effect may be confounded by these order effects rather than being a direct result of the treatment itself.

The other options each point to different aspects of experimental design. One option mentioning the required time to conduct the study does not capture the essence of repeated measures, as they can sometimes be more time-efficient due to fewer participants needed. Another choice discusses the control of variables, suggesting a lack of control which is not inherently a flaw of the repeated measures design, as it often does help control for individual differences. The last option implies that each participant performs only one condition, which contradicts the very nature of a repeated measures design, where participants partake in all conditions. Thus, focusing on the order effects succinctly encapsulates this critical drawback