The correct answer is option 1: Rarefied air.

Leh is a high-altitude region, located in the foothills of the Himalayas. Due to its high altitude, the air in Leh is rarefied, meaning it has lower atmospheric pressure and contains less oxygen compared to lower altitudes. This is because the density of air decreases as altitude increases.

When a person sleeps with their head in the open and their legs in the shade of a tree, two different conditions are present simultaneously. The person`s head in the open is exposed to direct sunlight, which can lead to sunstroke. Sunstroke occurs when the body becomes overheated due to prolonged exposure to high temperatures.

On the other hand, the person`s legs in the shade are exposed to the cold temperatures typical of high-altitude regions. These low temperatures can cause frostbite, which is the freezing of body tissues.

The combination of rarefied air, high heat from the sun, and low temperatures can make a person in Leh susceptible to both sunstroke and frostbite simultaneously. It is important to take precautions and be aware of the potential risks when staying in high-altitude regions like Leh.