Vitamin $B_{12}$is a complex of

Vitamin $B_{12}$is a complex of cobalt(III) ions.

Cobalt is part of the vitamin $B_{12}$complex. It's cyanocobalamin($C_{63}{H}_{88}Co{N}_{14}{O}_{14}P$). Meat, fish, eggs, milk, etc. are some of its main sources. Pernicious anemia results from a deficiency in it.

Hence, option B is correct.