Answer:
Testing
Step-by-step explanation:
Internal validity can be defined as the approximate truth when it comes to making inferential deductions about casual or cause and effect relationships. In this scenario, the likely threat that would make this hypothesis wrong is that testing, and this is because once something has been done once, there is every tendency that one would improve on it when the same thing is done a second time as a result of being exposed to it twice. Hence the researcher inferring that jumping jacks will improve his students' quiz performance in such scenario might be wrong.