The sigmoid nature of Hb-O2 dissociation curve is because of:
Ans. is 'a' i.e. Binding of one O2 molecule increases the affinity for next Hb also .Lehninger writes : "Hemoglobin must bind oxygen efficiently in lungs and release oxygen in the tissues, myoglobin, or any protein that binds oxygen with a hyperbolic binding curve, would be ill-suited to this function. A protein that binds O2 with high affinity would bind it efficiency in the lungs but would not release much of it in the tissues. If the protein oxygen with a sufficiently low affinity to release it easily in the tissues, it would not pick up much of oxygen in lungs.Haemoglobin solves this problem by undergoing a transition from a low affinity state (the *T state) to a high affinity state (the (R state) as more oxygen molecules are bound. As a result, the Haemoglobin hos a hybrids-shaped, or sigmoid*, binding curve for oxygen*. "
Binding of one O2 molecule increase the affinity for the next O2 molecule.
Alpha chain has more affinity for O2 than beta chain
Beta chain has more affinity for than alpha chain
Hemoglobin in acidic in nature.
Principles of Gas Exchange -  Respiratory Membrane