Answer:
Blood vessel diameter
Step-by-step explanation:
A blood vessel is an essential part of the circulatory system that is responsible for the transportation of blood in the human body.
The effect of blood vessel diameter on blood flow resistance is inversely related. For example, given equal volume of blood, the increase in the diameter means the blood flows easily, with less contact with the blood vessel, hence low blood flow resistance. The decrease in the diameter, means the blood doesn't flow easily, with more contact with the blood vessel, hence a high blood flow resistance.
Therefore, the most significant source of blood flow resistance is blood vessel diameter.