Answer:
C. Neither taking vitamin C nor the immune system response
Step-by-step explanation:
A control variable, also known as a constant variable, is a factor in an experiment that is intentionally kept consistent and unchanged throughout the study. Its purpose is to provide a baseline for comparison, ensuring that any observed effects can be attributed to the manipulated variable.
In the given scenario, the control variable would be a factor that remains constant and unaffected by the study. Among the options provided, option C. Neither taking vitamin C nor the immune system response is the control variable because it represents a scenario where neither the intake of vitamin C nor the immune system response is manipulated or changed.