Coenzyme B12, also known as adenosylcobalamin, is a cofactor that is involved in a variety of enzymatic reactions. It contains a cobalt ion at its center, which is coordinated to a number of ligands. Among these ligands, the group that is bound to the metal ion in coenzyme B12 is adenosyl.

Explanation:

Coenzyme B12 is a complex molecule that plays an important role in a number of enzymatic reactions. It is involved in the synthesis of amino acids, nucleotides, and fatty acids, as well as in the metabolism of carbohydrates and fatty acids. Coenzyme B12 contains a central cobalt ion that is coordinated to several ligands, including:

• Adenosyl group: This group is derived from adenosine, a molecule that is found in many important biological molecules, such as ATP and DNA. The adenosyl group is bound to the cobalt ion through a covalent bond.

• Cyanide group: This group is bound to the cobalt ion through a covalent bond. It acts as a stabilizing ligand, helping to keep the cobalt ion in the correct oxidation state.

• Methyl group: This group is bound to the cobalt ion through a covalent bond. It is involved in the transfer of methyl groups between molecules.

• Hydroxyl group: This group is bound to the cobalt ion through a covalent bond. It is involved in the transfer of protons between molecules.

Out of these ligands, the adenosyl group is the one that is bound to the metal ion in coenzyme B12. It is involved in a number of enzymatic reactions, including the conversion of methylmalonyl-CoA to succinyl-CoA, which is important for the metabolism of fatty acids and amino acids.

In conclusion, the correct answer is option 'C', adenosyl, as it is the group that is bound to the metal ion in coenzyme B12.