That would be C. The control group is the group where no variable has been changed for it and it’s how something normally would be.
For example if we’re experimenting the effect of excess water on plant growth the plant getting normal amount of water is the control group and the plant getting excess water is the test group(or groups in most cases there is more than one).
You can use the growth of the control plant as a reference of how the growth of the other plant has been affected(wether it has been increased or decreased or stayed the same)