Final answer:
The biologically active form of vitamin D is 1,25-dihydroxyvitamin D, also known as calcitriol, which is produced through a process in the liver and kidneys.
Step-by-step explanation:
The biologically active form of vitamin D is known as calcitriol. This active form is produced in a two-step process involving the liver and the kidney. Initially, vitamin D3, also called cholecalciferol, is synthesized in the skin upon exposure to sunlight. Then, the liver converts it into 25-hydroxyvitamin D (25(OH)D) also known as calcidiol.
Finally, in the kidneys, calcidiol undergoes another hydroxylation reaction to form 1,25-dihydroxyvitamin D (1,25(OH)₂D or calcitriol), which is the most potent and active form of vitamin D in the body. Calcitriol plays a crucial role in calcium and phosphorus metabolism promoting their absorption in the intestines and maintaining proper bone health. Thus, the correct answer to the question is D. 1,25-Dihydroxyvitamin D.