Which factors improve the likelihood of venous access?

B is the correct choice because nitropaste, which contains nitroglycerin, acts as a vasodilator. When applied to the skin, it helps to widen blood vessels, thereby increasing blood flow to the area. This enhanced circulation can significantly improve the likelihood of successful venous access by making the veins more prominent and easier to locate and puncture.

In contrast, cold room temperature can cause vasoconstriction, which reduces blood flow to the extremities and makes venous access more challenging. While a standing position might make certain veins more visible, it does not necessarily enhance venous access in the same way that the application of nitropaste would. Furthermore, using cool compresses may also lead to vasoconstriction, inhibiting blood flow and making access less favorable. Thus, nitropaste's role as a vasodilator clearly stands out as a factor that actively improves the chances of venous access.