The respiratory pigment present in the RBC of vertebrate blood is called
a) Haemoglobin
b) Haematin
c) Haematochrome
d) Erythrocyanin

Hint: Oxygenated hemocyanin is blue in colour, deoxygenated is colourless. Haematin is dark bluish or pigment containing iron in the ferric state, obtained by the oxidation of haem. Hematochrome could be a yellow, orange, or red pigment present in some alga. Hematochrome may be a mixture of carotenoid pigments and their derivatives.

Complete answer:
In order to answer this question we need to have knowledge about respiratory pigments.
Respiratory pigment could be a substance that encompasses a molecule of protein and a pigmented prosthetic group. It is required to hold oxygen to the blood and circulate to the various body parts. The deoxygenated blood is converted into oxygenated blood, and dioxide is released. It has affinity for oxygen and binds strongly with it, forming oxyhemoglobin. It helps to move oxygen from lungs to tissues. Most such pigments are present within the blood, transporting oxygen within the cardiovascular system from the respiratory organs to the tissues of the body;. In vertebrates the respiratory pigment is haemoglobin, contained within the erythrocytes (red blood cells). Red blood cells, also observed as red blood corpuscles, haematodes, erythroid cells or erythrocytes, are the common variety of corpuscle and also the vertebrate's principal means of delivering oxygen to the body tissues—via blood flow through the vascular system. Hemoglobin is spoken as a respiratory pigment since it's accountable for the red color of RBCs and muscles. it's bright red when oxygenated, and redness when deoxygenated. Haemoglobin is an iron-containing protein present in erythrocytes of vertebrates. Each hemoglobin binds to four oxygen molecules and carries oxygen from lungs to tissues. It also helps within the transportation of carbonic acid gas, hydrogen ions back to the lungs from the tissues.
Therefore, the correct answer is a, haemoglobin.


Note: Most of the pigments are present in blood, exception is myoglobin, which occurs in the muscles. It is only found in the bloodstream after muscle injury.