Which of the following is the major circulating form of Vitamin D?

25-(OH) cholecalciferol is the major circulating form of vitamin D because of the half-life of 3-weeks 1,25-dihydroxy vitamin D3 a.k.a. calcitriol is the biologically active form vitamin D. But, it's half life is low. In clinical laboratory, we measure 25-(OH) cholecalciferol.