Any uncontrolled variable that changes systematically with the independent variable is:

I believe the answer is: . a confounding variable

Confounding variables refers to the variables that does not beling to either the dependent or independent variable groups, but their existence could affect the outcome of the researchs.
Confounding variables tend to make the data become invalid/inadmissible because researchers wouldn't know whether their control groups played any factors in influencing the outcome.