Haemocyanin, the blue colouring pigment of molluscan blood contains Magnesium, Copper, Iron, Manganese.

Haemocyanin is the pigment found in the blood of molluscs, crustaceans and arthropods. The active centre of haemocyanin contains two molecules of copper which bind to a single oxygen molecule. The blue colour of this pigment is due to the presence of copper which is blue in colour. So, the correct option is Copper.