Answer:
The group size is the independent variable.
Step-by-step explanation:
In an experiment the independent variable is one that is influencing a result. That is, it is the variable that relates a situation to what will be evaluated and that is not influenced by other variables so that it can work. It is the independent variable that "dictates" what will be measured in the survey, we can say. In the case shown in the question above, the research sought to assess the influence of the size of different groups on puzzle solving. Since group size is the main factor influencing puzzle solving, group size is the independent variable.