Altitude sickness, also known as acute mountain sickness (AMS), occurs when individuals travel to high altitudes. The correct answer is option-2: low partial pressure of oxygen. At higher altitudes, the air becomes thinner and the partial pressure of oxygen decreases. This leads to a lower concentration of oxygen in the bloodstream.

Option-1, high partial pressure of oxygen, is incorrect. High partial pressure of oxygen is found at lower altitudes where there is a higher concentration of oxygen in the air.

Option-3, low level of haemoglobin, is not the direct cause of altitude sickness. However, individuals with pre-existing medical conditions like anemia or low levels of haemoglobin may be more susceptible to developing altitude sickness.

Option-4, high partial pressure of carbon dioxide, is also incorrect. Altitude sickness is primarily caused by lower levels of oxygen, not higher levels of carbon dioxide.

In conclusion, altitude sickness occurs due to the low partial pressure of oxygen at high altitudes, resulting in a decreased oxygen supply in the bloodstream.