Which of the following best defines a controlled experiment?

A controlled experiment is best defined by the manipulation of variables. This means that in such an experiment, a researcher deliberately changes one or more factors, known as independent variables, to observe the effect of this change on other factors, referred to as dependent variables. The purpose of this manipulation is to establish a cause-and-effect relationship between the variables being tested.

In a controlled experiment, while it's common to keep other factors constant to isolate the effects of the manipulated variables, the emphasis lies on the active manipulation of those variables. By doing so, researchers can confidently attribute any observed changes in the dependent variable directly to the changes made in the independent variable.

The other options describe aspects or characteristics related to experiments, but they do not encapsulate the defining feature of a controlled experiment as effectively. For instance, the ability to repeat an experiment (first choice) is crucial for scientific validation but does not necessarily characterize the nature of a controlled experiment itself. Keeping factors constant (third choice) is an important practice in experimental design but is more about how experiments are conducted rather than what defines them. Lastly, testing multiple hypotheses (fourth choice) might apply to various experimental setups but does not specifically relate to the definition of controlled experiments, which typically focus on a clear relationship between