Detailed Solution:
The correct answer is “B12”.
The vitamin which contains cobalt is: (4) B12
1) Vitamin B1 (Thiamine): Important for energy metabolism, doesn't contain cobalt.
2) Vitamin B2 (Riboflavin): Essential for cellular function, doesn't contain cobalt.
3) Vitamin B6 (Pyridoxine): Involved in protein metabolism, doesn't contain cobalt.
4) Vitamin B12 (Cobalamin): The only B vitamin that contains cobalt as a central element in its structure. It's vital for red blood cell formation and nervous system function.