The source book printed no answer; this one was worked out and checked — see the explanation.
Explanation
Ni is the metal of cofactor F430 in methyl-coenzyme M reductase, which releases methane; Zn is at the active site of carbonic anhydrase, which hydrates CO₂ to H₂CO₃; Mo is in xanthine oxidase, which oxidises xanthine to uric acid. Hence A-iii-Y, B-i-Z, C-ii-X.