Independent variables are best defined as:

Independent variables are best defined as factors that can be manipulated within the context of an experiment or study. They represent the conditions that researchers alter to observe the effects on dependent variables, which are the outcomes or responses measured in the experiment. The ability to manipulate independent variables allows researchers to establish cause-and-effect relationships by demonstrating how changes in these variables lead to different outcomes.

For instance, in a study examining the effects of different teaching methods on student performance, the teaching methods would be the independent variables, as the researcher can change these methods to see how they influence student outcomes. This manipulation is crucial for testing hypotheses and gaining insights into the relationships between various factors.

The other options describe characteristics that do not accurately capture the essence of independent variables in research. Such distinctions help clarify the role independent variables play within the research framework.