Answer:
C) control
Step-by-step explanation:
The control group can be explained as the group in an experiment or study that does not receive treatment by the researchers and is then used as a benchmark to measure how the other tested subjects do.
This group is used in an experiment or research as a way to ensure or know that your experiment actually works. In other words, it is used in an experiment to establish its validity.
In this case, the participants that were told to solve the problems alone, were in the control group. And they were used to establish the validity of the study.