What aspect of a study ensures that measures yield consistent results over time?

The correct choice is focused on reliability, which refers to the consistency of a measure over time. When a study has high reliability, it means that if the same measurements were taken under the same conditions, the same results would be achieved. This is crucial in psychological research as it enhances the credibility of the findings; researchers need to know that their measurements can be trusted and will produce similar outcomes regardless of when they are administered or by whom.

In contrast, flexibility pertains to how adaptable a study may be in its approach or design, but it does not ensure results are consistent. Accuracy refers to the correctness of the results or measurements in reflecting the true value, whereas validity relates to the degree to which a test measures what it claims to measure. Both accuracy and validity are important, but they do not specifically address the consistency of results over time, which is the essence of reliability. Therefore, reliability is the key concept that ensures that the measures yield consistent results across different occasions.