The natural services that support life and human economies at no monetary cost, such as pollination
and the purification of air and water
Chapter 1 page 40 the environment and sustainability

answer: ice crystals in air

the water from the water body or another source of water gets evaporated as water vapor as an effect of heat on water. these water vapor rises in the upper layers of the earth atmosphere under high atmospheric pressure. the atmospheric pressure of these water molecules starts to decrease due to the effect of cold air. the invisible water vapor condenses into visible water droplets and ice crystals.these ice crystals and water droplets being less denser than the surrounding air floats on air as clouds.

since there are no options posted i'll give you a few examples.

a dolphin's flipper, bird's wing, cat's leg.

bacterium is not correct