Final answer:
The independent variable in the study between groups who exercise and do not exercise is the act of exercising, which researchers manipulate to observe its effects on weight loss.
Step-by-step explanation:
The independent variable in the experiment between Group A, which decides to exercise for a month, and Group B, which decides not to exercise, is the act of exercising itself. This is what the researchers have control over and are actively changing to observe its impact on the dependent variable, which is the amount of weight lost by each group. In scientific studies, the independent variable is the factor that is manipulated or changed while the dependent variable is what is being measured. Thus, the exercise status (whether the group exercises or not) is what differentiates the experimental group from the control group.