The correct answer is C. Experiments.
An experiment is a research method in which a researcher manipulates one or more variables to observe their effects on other variables. The researcher then compares the results of the experiment to a control group, which is a group of participants who do not receive the treatment. This allows the researcher to determine whether the changes in the experimental group were caused by the treatment or by other factors.
Observations are a research method in which a researcher collects data by watching people or events. The researcher does not control the situation or the people being observed.
Surveys are a research method in which a researcher collects data by asking people questions. The researcher may ask people questions in person, over the phone, or online.
Interviews are a research method in which a researcher collects data by talking to people in person or over the phone. The researcher asks people questions about their experiences, opinions, or beliefs.
In conclusion, experiments are the only research method that involves manipulating one or more variables to observe their effects on other variables.