Final answer:
The question relates to physiological biology and the constant factor during the activity is stroke volume, as there are no indications of changes to it in the provided context, making the correct option C. stroke volume.
Step-by-step explanation:
The question seems to pertain to a physiological biology activity, likely focusing on cardiovascular dynamics or fluid dynamics where variables affecting the flow of fluid through a system (in this case, potentially blood through blood vessels) are manipulated to observe different outcomes. In this scenario, when analyzing factors that stayed constant during the activity, we want to identify the variable that did not change, no matter the manipulations made to the other factors. Considering the context provided, none of the scenarios involved changes to stroke volume, which directly relates to the amount of blood pumped with each beat, and it wasn't mentioned as a variable factor in the various tests.
So, given the options A. pump rate, B. right flow tube radius, C. stroke volume, and D. flow rate, and the lack of information suggesting that stroke volume changed during the activity, the correct option is C. stroke volume. The flow rate is indeed affected by various factors like pressure, viscosity, length of the tube, and radius, as outlined in the examples provided, but these do not include stroke volume, indicating that it remained constant.