Final answer:
High clouds are made mainly of ice crystals and low clouds of water droplets due to temperature differences with altitude; temperatures below 0°C at higher altitudes lead to ice crystal formation.
Step-by-step explanation:
High clouds consist mainly of ice crystals, while low clouds consist mainly of water droplets because of the differences in temperatures at various altitudes. In the troposphere, the lowest layer of Earth's atmosphere, temperatures typically decrease with altitude. The correct statement explaining why high altitude clouds are icy is option D: 0 km to 3.2 km is above 0°C, where water droplets exist as a liquid in low clouds, whereas the average air temperature at altitudes from 6.0 km to 16.0 km is at or below 0°C, leading to the formation of ice crystals in high clouds. This is further explained by the concept of supercooling, where water droplets can exist in a liquid state even below freezing point until they reach about -10°C, at which point they can freeze into ice crystals, which are more likely at higher cold altitudes.