Which of the following best describes a controlled experiment?

A controlled experiment is characterized by the meticulous maintenance of all variables except for the one being tested. This means that researchers isolate the independent variable—what they are deliberately changing—and keep everything else constant, including environmental factors and participant characteristics, to ensure that any observed changes in the dependent variable can be attributed solely to the manipulation of the independent variable.

By controlling for extraneous variables, the integrity of the experiment enhances, leading to more reliable and valid conclusions. This approach minimizes potential confounding influences, allowing researchers to determine cause-and-effect relationships more effectively.

In contrast, the other options do not accurately reflect the principles of a controlled experiment. Observing existing conditions does not involve manipulation and therefore lacks the experimentation aspect. Allowing variable conditions to fluctuate introduces confusion into the experiment and can lead to ambiguities when interpreting outcomes. Introducing multiple independent variables complicates the experiment and makes it challenging to ascertain which variable is responsible for any observed effect. Thus, the key to a controlled experiment lies in keeping all other variables constant while changing just one.