Correct Answer: A
Explanation:
Vitamin B12 consists of a class of chemically related compounds (vitamers), all of which have vitamin activity. It contains the biochemically rare element cobalt sitting in the center of a planar tetra-pyrrole ring called a Corrin ring. Biosynthesis of the basic structure of the vitamin is accomplished only by bacteria and archaea (which usually produce hydroxocobalamin), but conversion between different forms of the vitamin can be accomplished in the human body. A common semi-synthetic form of the vitamin is cyanocobalamin, which does not occur in nature but is produced from bacterial hydroxocobalamin. Because of its stability and lower production cost, this form is then used in many pharmaceuticals and supplements as well as a food additive.