The correct answer is option 3 - Cobalt. Cobalt is the metal ion that exists in Vitamin B12. Vitamin B12, also known as cobalamin, is a complex molecule that contains a cobalt ion at its center. The presence of cobalt in Vitamin B12 is essential for its biological activity. Cobalt is involved in important functions in the body, such as the synthesis of DNA and the metabolism of fatty acids and amino acids. Iron, option 1, is an important nutrient, but it is not the metal ion present in Vitamin B12. Zinc, option 2, and Nickel, option 4, are not the correct metal ions either. It is worth noting that while other metals may bind to Vitamin B12 in laboratory settings, cobalt is the specific metal ion found naturally in this vitamin.