Which of the following is a constituent of vitamin B12?

Cobalt is a constituent of Vit B12. Vitamin B12 is water soluble, heat stable and red in colour. It contains one cobalt atom, Four pyrrole rings co-ordinated with a cobalt atom is called a Corrin ring. The 5th valency of the cobalt is covalently linked to a substituted benzimidazole ring. This is then called cobalamin. The 6th valency of the cobalt is satisfied by any of the following groups: cyanide, hydroxyl, adenosyl or methyl Ref: DM Vasudevan - Textbook of Biochemistry, 8th edition, page no: 477