The metal ion present in vitamin B12 is cobalt. Cobalt is a transition metal element that is essential for the synthesis of vitamin B12. It plays a crucial role in the structure and function of the vitamin. Cobalt is able to form coordination complexes with organic molecules, and in the case of vitamin B12, it forms a complex with a corrin ring structure. This cobalamin complex is essential for various biochemical reactions in the body, including the conversion of homocysteine to methionine and the synthesis of DNA. While the other options (zinc, nickel, and iron) are also transition metals, they are not the metal ion present in vitamin B12. Therefore, the correct answer is option 1: cobalt.