Explanation

Haemoglobin is correct because it contains the iron-containing heme group which requires cobalt for its structure/function (though iron is the primary metal often associated with hemoglobin, B12 also contains Co). Vitamin K is incorrect as it does not contain cobalt. Chlorophyll is incorrect as it primarily contains magnesium. Vitamin B12 is incorrect because while it *contains* cobalt, haemoglobin's structural association in blood components makes D a more definitive answer if the question implies major biological sources or complexes.