Researchers randomly choose two groups from 15 volunteers. Over a period of 9 weeks, one group watches television before going to sleep, and the other does not. Volunteers wear monitoring devices while sleeping, and researchers record dream activity. Which type of study method is described in each situation?
A. Not enough information given.
B. observational study
C. survey
D. controlled experiment

B. In observational study, the researcher has no control over the variables and no treatment is applied on the variables. But in case 1, only one group is allowed to watch television (controlled) and in case 2 volunteers wear monitoring devices while sleeping ( treatment is applied ) so not observational study

C. Survey study includes collecting the information or data as reported by the individuals but in the given situations the volunteers are not reporting any information to the researchers instead they are monitored and controlled by the researchers. So, not survey study

D. A controlled experiment is one in which the variables are manipulated by the researchers and here in all the cases the researchers are controlling the volunteers in one way or other so in both the situations controlled experiment study is described

Hence, the option D is appropriate.