Answer:
The correct answer is the selection attrition threat.
Step-by-step explanation:
In investigations involving two or more groups, they may encounter difficulties with internal validation threats.
For example, in this case, there is a validation threat that is the selection attrition threat. When a selection attrition risk occurs, the internal validity of the investigation is affected because the participants leave the process before the study ends.
And in this study, the results were similar, this is because in one of the groups, they abandoned members, and this leads to the validity of the study is at risk.
I hope this information can help you.