Answer:
The answer is experiment.
Explanation:
An experiment, or experimental research, is a research method in which the researcher manipulates or changes the value of one variable, (the independent variable) and measures the variable that is considered the dependent variable, in order to find how the change affects it.
The other three posible answers, observational research, survey and case study are also research methods, but the researcher can't manipulate or control any of the variables, these methods are used only to measure a specfic variable.
