Final answer:
Clouds stay aloft because they are less dense than air.
Step-by-step explanation:
The reason clouds stay aloft is because they are less dense than air. Option a) is the correct answer. Clouds are made up of tiny droplets of water or ice crystals that are lighter than the surrounding air. This difference in density causes the clouds to float in the atmosphere.
Just like helium-filled balloons float in the air because the density of helium is less than the density of air, clouds float because they are less dense than the air they are surrounded by. The buoyant force exerted by the air on the cloud particles helps them stay aloft.
Contrary to options b) and c), clouds are not electrically charged and they are not anchored by invisible tethers. Additionally, clouds do not contain helium, so option d) is also incorrect.