Ans. is 'b' i.e., 25 hydroxyvitamin DThough, 1, 25 dihydroxyvitamin D (calcitriol) is the active form of vitamin D, its serum measurement does not provide any information about vitamin D status because it is often normal or elevated due to secondary hyperparathyroidism associated with vitamin D deficiency...Measurement of 25 hydroxyvitamin D (calcifedial) in serum provides more accurate information of vitamin D status. The usual values are :-< 20 ng/ml - Deficiency20-29 ng/ml - Insufficiency30-100 ng/ml - Normal> 100 ng/ml - Toxicity