The correct answer is - b. clouds.
One of the interacting parts in a weather system are the clouds. The clouds themselves form from the water vapor that goes up in the atmosphere, and when it reaches certain height where the temperature is cooler, it starts to condensate and turn into a liquid or frozen state, thus forming the clouds. From the clouds we get the rain, the snow, hailstorm, which are all essential parts of the weather on our planet.