Final answer:
The type of blood vessel with a thick wall containing smooth muscle that helps regulate blood pressure is an artery (a). Arteries have thicker walls than veins due to the high pressure of blood flow from the heart, and their ability to contract and relax is vital for blood pressure regulation.
Step-by-step explanation:
The type of blood vessel that has a thick wall with a lot of smooth muscle to help regulate blood pressure is an artery (a). The walls of arteries are much thicker due to the higher pressure of blood that flows through them as compared to veins. Arteries are equipped with thick smooth muscle layers and elastic connective tissue that enable them to accommodate changes in pressure from the heart. This structural trait allows arteries to contract and relax, also known as vasoconstriction and vasodilation, which helps in regulating blood pressure. In contrast, veins (v) have thinner walls as they carry blood back to the heart at a lower pressure and utilize valves to prevent backflow.