Explanation:

When it comes to water droplets in the atmosphere, their size and weight determine their behavior. The behavior can range from floating to falling as precipitation. In this case, the correct answer is option 'C', which is rain. Let's explore why rain droplets are bigger and heavier compared to fog and clouds.

Fog:
- Fog consists of tiny water droplets suspended in the air near the ground.
- These droplets are very small and light, typically less than 0.02 mm in diameter.
- Due to their small size and low weight, fog droplets are able to float in the air rather than falling to the ground.
- Fog usually forms when the air near the ground is cooled to the point where it becomes saturated with water vapor, causing condensation.

Clouds:
- Clouds are similar to fog but are formed higher in the atmosphere.
- Cloud droplets are also very small, typically around 0.01 mm in diameter.
- Like fog, cloud droplets are light and able to float in the air.
- Clouds form when warm air rises and cools, causing the water vapor in the air to condense into tiny droplets or ice crystals.

Rain:
- Rain droplets, on the other hand, are much larger and heavier compared to fog and cloud droplets.
- Rain droplets can range in size from 0.5 mm to several millimeters in diameter.
- The larger size and weight of rain droplets cause them to fall from the sky as precipitation.
- Rain occurs when the cloud droplets grow in size through collision and coalescence or by the process of ice crystal aggregation.
- Eventually, the raindrops become too heavy to be suspended in the air, and gravity pulls them down towards the ground.

Smoke:
- Smoke particles are not water droplets but tiny solid or liquid particles produced by combustion.
- These particles are usually much smaller and lighter than water droplets, and they can also float in the air.
- However, smoke particles do not have the same behavior as water droplets, and they do not form clouds or precipitation.

In summary, while fog and clouds consist of small and light water droplets that can float in the air, rain droplets are much bigger and heavier, causing them to fall as precipitation. Smoke particles, on the other hand, are not water droplets and have different properties.