C. variable
A variable is a factor such as a person's age, weight, or environment that can change and influence the results of a study. It is a measurable characteristic or trait that can vary between individuals or groups and can be manipulated or controlled in an experiment. Variables are important in scientific research as they allow researchers to understand how different factors may be related to the outcome of the study. For example, in a study on the effects of a new medication, the variable could be the medication, while the age and weight of the participants would also be considered as variables.