I think the correct answer would be the last option. A kind of variable that a researcher purposely changes in an investigation is the independent variable. It is the value that does not depend to any change in any variable involved. Instead, it is the one that affects the other variable called the dependent variable. The dependent variable is the variable that is being tested or studied. Also, along with these two there are variables that are held equal all through out the research. These are called the constants. These constants are factors that might affect the dependent variable that is why they are held constant to prevent more errors.
