One more chemistry question..?

Without doing the calculations for the exact numbers, you can figure this out by just looking at the periodic chart and finding the atomic weights for each metal, since the molar concentration will be the mg/L divided by the atomic weight. So the one with the higher mg/l and lower atomic weight will have the highest molar concentration. The atomic weights of Fe, Mn, Zn and Cu are very close to each other, but Cu at 63.5 and 0.5 mg/L will give you the highest M.