The correct answer is Option 4: density.

Clouds are formed when water vapor in the atmosphere condenses into tiny water droplets or ice crystals. These droplets or crystals are lighter than the surrounding air, allowing them to float or suspend in the atmosphere. The primary reason for this suspension is the difference in density between the cloud particles and the surrounding air.

Density is defined as the mass of an object per unit volume. In the case of clouds, the density of the water droplets or ice crystals is lower than that of the surrounding air. This lower density causes the cloud particles to be buoyant, allowing them to remain suspended in the atmosphere.

To further understand this concept, let`s consider the process of cloud formation. When warm, moist air rises, it expands and cools as it reaches higher altitudes. As the air cools, it reaches a point called the dew point, where it becomes saturated and can no longer hold all the water vapor it contains. At this point, the excess water vapor condenses into tiny droplets or ice crystals, forming clouds.

These cloud particles are tiny and have a relatively low mass compared to the volume they occupy. The surrounding air, on the other hand, is composed of nitrogen, oxygen, and other gases, which have a higher density compared to the cloud particles. As a result, the cloud particles, being less dense, remain suspended in the air instead of falling to the ground.

Temperature, velocity, and pressure are not the primary factors determining why clouds float in the atmosphere. While temperature and pressure variations do play a role in cloud formation and stability, they do not directly contribute to the buoyancy of clouds. Similarly, velocity, or the speed of air currents, can influence the movement and shape of clouds but does not determine their ability to float.

In summary, clouds float in the atmosphere due to the difference in density between the cloud particles and the surrounding air. This difference allows the cloud particles to be buoyant and remain suspended in the atmosphere, creating the beautiful formations we observe in the sky.