The correct option is A Possess more RBCs with haemoglobin of low O2 binding affinity
When one is at a high altitude, he/she has to acclimatize to it. Breathing becomes difficult at high altitudes, because of the low density of the air. There is not sufficient oxygen in the atmosphere to oxygenate the blood adequately. Oxygenation of blood depends upon the partial pressure of oxygen in the atmosphere. As atmospheric pressure decreases, so does the partial pressure of the oxygen. For short durations, humans compensate for high altitudes by increasing the respiratory rate (hyperventilation). During a longer period of acclimatization, physiological changes occur including increased levels of haemoglobin. Other changes in the body's acid-base balance can also happen.
Hence, the correct answer is option (A).