Several days into the study, some of the students in the math-based reading program were getting frustrated by the frequent questions from their teacher. The students wanted to see the story in the books progress quickly, and they began acting out. The teacher decided it was best to either let these students join the control reading program or to try reading on their own in a different part of the classroom. Thus these students no longer participated in the math-based reading program. The researchers found that students in the math-based reading program performed much better on the PICO post-test than students in the control reading program. Which internal validity threat is most likely occurring in this scenario

The internal validity threat that is most likely occurring in this scenario is:

Selection threat.

Explanation:

Selection threat or bias results from how groups were selected for the test, especially when the different groups were not comparable before the study.  For example, some of the students were frustrated by the frequent questions from their teacher.  They wanted quick progress, and as a result, started acting out.  Grouping these students together as a control group creates a threat called a selection bias or selection threat.  It shows that any other factor (reading on their own and no participation in the math-based reading program) than the reading program leads to the post-test differences between the groups.