Which of the following describes an independent variable?

An independent variable is defined as a factor or variable that is manipulated or changed by the researcher in an experiment in order to observe its effect on another variable, known as the dependent variable. Specifically, it is the independent variable that is hypothesized to influence or affect the outcome of the dependent variable. This concept is fundamental in experimental design, as it allows researchers to establish cause-and-effect relationships.

The options present other types of variables, which are important to understand in the context of research but do not accurately describe the role of the independent variable. For instance, the variable that is measured during the experiment is indicative of the dependent variable, as it reflects the outcomes of changing the independent variable. Meanwhile, a variable that stays constant throughout the experiment refers to a controlled or constant variable, which is kept unchanged to ensure that the results are attributed to the manipulation of the independent variable. Lastly, while a variable controlled by the researcher may overlap with the concept of an independent variable, it doesn't specifically capture the essential characteristic that the independent variable actively affects the dependent variable. Thus, the correct identification of an independent variable is its role in influencing other variables within the experimental framework.