The correct option is C Iron
RBCs contains the respiratory pigment, haemoglobin. Haemoglobin is the iron-containing metalloprotein in the RBCs of almost all the vertebrates. In haemoglobin, four atoms of iron are present, which can be in an oxidised state, i.e., with an atom of oxygen attached to it, or in a non-oxidised state.
Thus, the correct answer is option B.