An experiment has internal validity when:.
a. the dependent variable is measured reliably
b. the dependent variable is valid
c. only the independent variable could have caused the results
d. the independent variable is strongly manipulated

c. only the independent variable could have caused the results

Explanation:

Internal validity is a term in research methodology which explains the extent to which a study reach conclusion in a trustworthy cause-and-effect relationship that exists between a treatment and an outcome, such that, only the independent variable could have caused the results

Hence, in a given study, to have an internal validity, the researchust establishes a conclusion in which there is no alternative explanations for a finding the effect aside the independent variable.

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