The curve is labeled as "B" is the curve that represents the changes in blood pressure throughout the circulatory system.
The curve that represents blood pressure changes throughout the circulatory system is the one that begins high on the left and decreases as it moves to the right, corresponding to the flow of blood from the aorta through the arteries, arterioles, capillaries, venules, veins, and finally to the venae cavae.
This is due to the fact that blood pressure is highest in the aorta shortly after the heart pumps it and lowers as the blood travels through the circulatory system, reaching its lowest point in the venae cavae just before the blood returns to the heart.