Answer:
c. The internal validity of the study
Step-by-step explanation:
The internal validity of a study is the term that refers to the ability of a study to show that it has a practical skill, through how well the results of that study have been established. in other words, this validity refers to a study that presents a good performance, because it was established with a correct design, well-developed variables, well-established results evaluation, among other factors, which show that the study was carried out correctly, so your results can be reliable.
If Dr. Kang decided not to use random assignment, the study's internal validity could be a threat to prove the truth, since we have little information on how the entire study was conducted.