Final answer:
The threat to internal validity in Dr. Feliciano's study is 'History', due to the influence of Halloween on children's activity levels during the study.
Step-by-step explanation:
In the study of the effects of sleep on children's activity level by Dr. Feliciano that takes place over several days, including Halloween, the threat to internal validity that has been confounded in this study is b) History. This term refers to events that occur during the course of the study which are outside the control of the researcher and can influence the results. In this case, Halloween could introduce changes to the children's activity levels that are not related to the amount of sleep they get, as the holiday typically involves increased excitement and activity such as trick-or-treating.