Hemocyanin is an oxygen-transport metalloprotein found in some invertebrate animals. It serves as their respiratory pigment, allowing them to transport oxygen throughout their bodies.

Option 1 states that hemocyanin contains one copper atom. This option is incorrect. Hemocyanin actually contains a larger number of copper atoms.

Option 2, which is the correct answer, states that hemocyanin contains two copper atoms. This is accurate. Hemocyanin molecules typically consist of two protein subunits, each of which contains a copper atom. These copper atoms are responsible for binding and transporting oxygen molecules.

Option 3 suggests that hemocyanin contains one iron atom, but this is incorrect. Iron is the metal found in the oxygen-binding protein hemoglobin instead.

Option 4 proposes that hemocyanin contains one magnesium atom. However, hemocyanin does not contain magnesium. Hemoglobin in some animals can have magnesium at its core, but not hemocyanin.

In summary, the correct answer to the question is option 2, which correctly states that hemocyanin contains two copper atoms.