Vasodilation and vasoconstriction are pressure dependent opening up and closing up of blood vessels. Via these important modalities, regulation of blood flows at various parts of the body is achieved.
Smooth muscle tissues have contractile fibrils that are not highly ordered. They occur in internal organs that are not directly under voluntary control. Their contractions and relaxations are important for vasoconstriction and vasodilation. Therefore, options A & B are incorrect because each of them forms only partial answers to the given question.
Venous walls are not thick and elastic and therefore, they do not undergo effective constriction and dilation. Therefore, Option D is incorrect.
Relaxation and contraction of the smooth muscles of the arterial wall enables the vasoconstriction and vasodilation that is observed in the blood supply. Contraction of the smooth muscles depends on calcium and phosphorylation of myosin (the contractile protein element). Relaxation is achieved by removing the conditions that lead to contraction.
Therefore, option (C) is correct.