The RBC count increases after an individual spends significant time at higher altitude because:

The correct option is D

Atmospheric $P{o}_2$ is lower at higher altitudes

In higher altitudes, the air pressure is lower, meaning that there are fewer molecules of oxygen present in the air. The body acclimatises to this in various ways, a major one of which is by increasing the number of red blood cells. This helps in distributing the available oxygen faster and more efficiently.