Answer:
The correct answer is The study has a problem with the External validity
Explanation:
External validity in research terms is the validity the research has in the outside world or in the applied social reality of the field of study, it is the extent in which the results and conclusions of the study can be applied and generalized to ohter external factors, therefore in this case, when the researchers did the experiment only with motivated volunteers did not take into account that people outside may not have the same motivation, therefore the results may not be the same in the external social reality and thus having problems with the external validity.
