"Sand in the scanty food, sand in the brackish water—water that was drunk lukewarm from a clammy, loathsome water-skin."
This is shows that nature is severe and unkind because sand is getting into the food and water. Also the water is being described as brackish. Brackish water is not pleasant to drink and it can sometimes contain salt water too.
"Sweating in the heat, we had lost the whole day, dreading to be buried alive in the drifting sand. "
This passage also shows the nature's unkindness and severity. It talks about how the heat is terrible and they risk the sands (or nature) swallowing them up.
The other passages are not correct. The first passage described the Arab Sheikh pressing them on...this is not nature. The third option describes nature as pleasant and enjoyable which is the opposite of severe and unkind.
