According to the sources, haemoglobin is a red-coloured, iron-containing complex protein found in erythrocytes (RBCs) . Its primary physiological role is to act as an oxygen carrier, binding reversibly with oxygen to form oxyhaemoglobin . Each haemoglobin molecule can transport a maximum of four molecules of O2, picking them up at the lung surface and dissociating at the tissues . Structurally, it represents a quaternary protein architecture composed of four subunits: two of the \alpha type and two of the \beta type . Other options are incorrect because globulins are involved in defence mechanisms, and hormones like insulin regulate blood sugar levels . Mastering these fundamental protein functions is a high-priority strategy, as NCERT textbooks serve as the definitive source for approximately 95% of NEET Biology questions .