Answer:
C. Vitamins
Step-by-step explanation:
The factor Vitamin is being tested in this experiment. For an experiment to occur, it is necessary that we plan an experimental group as well as a placebo group (control group). In this experiment, all other factors have the same experimental group as well as the same placebo group except for the factor vitamin. The factor 'vitamin' has a experimental group in which the particular vitamin is given to the object of test and a placebo group in which the vitamin is not given to the object of test. Hence, vitamin is the variable that is being tested in this experiment.