Which of the following types of validity is mainly concerned with the issue of…

Concept: In research methodology "validity" is a family of criteria, and each member answers a different question about a study. One member judges the quality of measurement, another judges the soundness of the inference drawn inside the study, and others judge how far the result travels beyond the study. A causal claim itself is warranted only when three conditions hold together: the presumed cause and the effect covary, the cause precedes the effect in time, and every rival explanation for that covariation has been ruled out.

Application: The criterion that governs the third condition — ruling out rival explanations within the study itself — is internal validity, so internal validity is the type mainly concerned with the issue of causality. A design scores high on it when random assignment, a control group, blinding and standardised procedures neutralise threats such as selection, history, maturation, testing, instrumentation, regression to the mean and attrition. Only then can the change recorded in the dependent variable be attributed to the manipulated independent variable.

Cross-check— set each type against the question it actually answers:

Result: Only internal validity is defined by the soundness of the cause-and-effect inference, so the answer is Internal Validity. Note the standard trade-off: tightening experimental control to raise internal validity often lowers external and ecological validity.