Given figure is a vein :
Veins are blood vessels that carry blood towards the heart. ... Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated blood to the heart. In contrast to veins, arteries carry blood away from the heart.
A: endothelium
B: connective tissue
C: smooth muscle
D: valve