Which factor distinguishes the experimental group from the control group

A control group is sort of the normal group. The group you aren't changing or messing with. It is to show what the species/experiment acts like on its own in response to something without you having to change it at all. 

With an experimental group, you are changing something about the subject before putting it in the conditions it will need to respond to. 

I don't entirely understand, but I know the control group is constant, unchanging. It sort of helps with distinguishing things. I have examples of experiments if that will help you understand what a control group is to answer the question. I also have a The Death Cure example if you have seen or read the book. I hope you do not fail this assignment, but even if I did not answer with an answer you were looking for, it could have helped you answer the question on your own. 

I saw somewhere a similar question, though I was given the answer choices. If this is the answer you are looking for: variable(s).

Control groups don't use variables because you aren't changing them.