Final answer:
Blood pressure in the vena cava (C) is the factor that least affects the stroke volume in the right ventricle, as venous pressure typically does not significantly hinder the heart's ventricular filling process.
Step-by-step explanation:
Stroke volume in the right ventricle is influenced by several factors, including filling time, compliance of the right atrium, blood pressure in the vena cava, systemic venous return, and pressure in the pulmonary artery. Among these, blood pressure in the vena cava (C) is the factor that least affects stroke volume in the right ventricle. This is because blood pressure within the venous system is normally relatively low. For blood to flow back into the heart, atrial pressure during diastole must be lower, usually approaching zero. Hence, venous pressure doesn't significantly hinder ventricular filling as it does in the arterial system, where high pressure can decrease flow.
The heart pumps blood from areas of higher pressure to lower pressure, with the ventricles ejecting blood during systole. The compliance of the right atrium, systemic venous return, and the pressure in the pulmonary artery directly relate to the ventricular filling and ejection of blood, thus impacting stroke volume considerably.