Answer: A. External Validity
Explanation: External Validity measures how well the result or outcome of a scientific study performs when applied to samples not used in the original study. It is a measure of well a scientific study generalize on similar samples outside the context of the the original experiment. It differs from internal validity which measures the approximate truth or validity of a research on the samples used for the study. External Validity is essential if a particular study would be considered in making global or large scale generalization.