At high altitude the partial pressure of oxygen is less than that at the ground level. This leads to

The atmospheric pressure around us is related to the partial pressure of oxygen. The atmospheric pressure exerted by the oxygen is less at high altitudes. Low concentration of oxygen in the blood and tissues of people living at high altitudes is due to low atmospheric pressure. As at high altitude the partial pressure of oxygen is less than at ground level, causing the low concentration of oxygen in the blood. Thus, (a) is the correct option.