To study the potential effects of social interaction on problem solving, some research participants were instructed to solve problems working together; other participants were told to solve problems working alone. Those who worked alone were assigned to the group.
A) experimental
B) survey
C) control
D) correlational

C) control

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.

Control

Explanation:

A control group is the group in a study that does not include the thing being tested and is used as a benchmark to measure the results of the other group (experimental group) and is one of the two groups in any valid experiment. The experimental group is the other one and is the group in which you are testing something. Control group consists of units of study that did not receive the treatment whose effect is under investigation