Final answer:
The active form of vitamin D is calcitriol, which is produced in the kidneys and plays a crucial role in regulating blood calcium levels.
Step-by-step explanation:
The active form of vitamin D is d) calcitriol. This is synthesized through a series of steps starting in the skin with the conversion of cholesterol to cholecalciferol (vitamin D3) when exposed to ultraviolet radiation. The liver then converts cholecalciferol to calcidiol.
The final step occurs in the kidneys, where calcidiol is transformed into calcitriol, the biologically active form of vitamin D that regulates blood calcium levels.