Give the reason for low concentration of oxygen in the blood and tissues of people living at high altitude

The correct option is Low atmospheric pressure.

Detailed Explanation:

At high altitudes, the atmospheric pressure is significantly lower than at sea level. Because the overall atmospheric pressure is lower, the partial pressure of oxygen (which is the pressure exerted by oxygen alone in the mixture of gases) also decreases proportionally.
According to Dalton's Law of partial pressures and Henry's Law of gas solubility, the rate of diffusion of a gas into a liquid (like blood in the lungs) is directly proportional to its partial pressure.

In the human respiratory system, oxygen diffuses from the alveoli in the lungs into the pulmonary blood capillaries. This diffusion is driven by the pressure gradient between the alveoli and the blood.
At high altitudes, the low partial pressure of alveolar oxygen reduces this pressure gradient. As a result, less oxygen diffuses into the blood, leading to a lower oxygen concentration (hypoxia) in both the blood and body tissues.