I would instantly rule out Survey and Seperation Study because 1) a survey is impossible in this instance, and 2) a seperation study will do nothing. It's just moving them apart from eachother. So that leaves Controlled Experiment and Observational Study. An observational study is when you have no control over the groups of subjects, nor what they are given. A controlled experiment is when you change only one aspect of what is given to the group. Since the only thing that is happening is the scientist is giving one rat a real pill, it would fall under controlled experiment. Thus, that is your answer.
Hope I helped ^.^