Final answer:
In an experiment, 'Unit sales' is not an independent variable; it is a dependent variable that is influenced by other factors such as price, distribution, and promotions.
Step-by-step explanation:
In a market research experiment, the independent variables are factors that the researcher can manipulate to examine their effect on an outcome. Among the options given, Price, Distribution outlet, and Free samples are all independent variables that can be changed by the researcher. However, Unit sales is not an independent variable but rather a dependent variable, as it is the outcome that is measured in response to changes in the independent variables. The dependent variable is influenced by and therefore depends on the independent variables.