Final answer:
Sophia should use mandatory stretch breaks as her independent variable, and she can choose either a within-subject or between-subject design depending on her experimental goals.
Step-by-step explanation:
In this case, the independent variable that Sophia should use in her field experiment is the mandatory stretch breaks. The independent variable is the factor that the researcher manipulates or changes in order to observe its effects on the dependent variable. The dependent variable, in this case, would be the employee morale that Sophia wants to improve.
In terms of whether the independent variable should be within or between subject, it would depend on the specific design of Sophia's field experiment. If she wants to compare the effects of mandatory stretch breaks on employee morale within the same group of employees, she would use a within-subject design where the same individuals are exposed to both the treatment (mandatory stretch breaks) and control condition (no mandatory stretch breaks).
On the other hand, if she wants to compare the effects of mandatory stretch breaks on employee morale between different groups of employees, she would use a between-subject design where different groups of employees are assigned to either the treatment or control condition.