What form of vitamin d is the major circulating form with a half-life of approximately 3 weeks?

The correct answer is calcifediol or calcidiol.
It is a prehormone that is produced in the liver. The mechanism of production includes hydroxylation of vitamin D3 (cholecalciferol) by the enzyme cholecalciferol 25-hydroxylase. Because it is the major form of vitamin D in the blood, it is considered the best indicator of vitamin D status. Calcidiol is converted in the kidneys into calcitriol which is the active form of vitamin D. 

C- a hypothesis or d- an observation