Final answer:
In the given research scenario, gender is a confounding variable because it is not manipulated by the researcher but may still affect the outcome of the experiment.
Step-by-step explanation:
In an experimental study, an independent variable is a factor that is manipulated or controlled by the experimenter to investigate its potential effect on a dependent variable, which is what the researcher measures to see how much effect the independent variable has. Given the scenario where a researcher is exposing subjects to different types of online messages and wants to know if gender plays a role in the effect of the exposure, gender would represent a variable that the researcher is not manipulating, but is observing its potential role or interaction with the research experiment. Gender in this context is neither the independent variable nor the dependent variable. It is not being manipulated, nor is it what is being measured as an outcome. Therefore, gender is C) A confounding variable, as it may interact with the independent variable and affect the dependent variable, playing a potential role in the research outcome that the researcher needs to account for.