Answer:
A. Experiment
Explanation:
The question above is related to the "Scientific Method." The "Experiment" is being conducted after the researcher comes up with a hypothesis (scientific guess). This step predicts the accuracy of the hypothesis and ensures that it is being conducted "fairly." In order to do this, the researcher has to control the situation. That said, he needs to manipulate one variable at a time. All the other conditions have to remain constant.
