What Does External Validity Refer To

What does external validity refer to?

A) the degree to which there can be reasonable certainty that the independent variables in an experiment caused the observed effects on the dependent variables
B) the degree to which there can be reasonable confidence that the same results would be obtained for other people and in other situations
C) the degree to which participants were assigned to the various conditions on the basis of representative criteria
D) the degree to which the experimental situation engages participants and leads them to behave naturally and spontaneously