Final answer:
Clouds are likely to develop at relative humidities (B) below 100% in the presence of hygroscopic nuclei.
Step-by-step explanation:
Where there is an abundance of hygroscopic nuclei in the atmosphere, clouds are likely to develop at relative humidities below 100%. Hygroscopic nuclei are small particles that water vapor can condense onto, which facilitates the formation of cloud droplets.
These nuclei can attract water in lower relative humidity conditions, which is why clouds can form even when the air is not completely saturated. When we talk about humidity, we are usually referring to relative humidity, which indicates how much water vapor is present in the air compared to the maximum amount the air can hold at that temperature.
As the air cools, it can hold less water vapor, and the relative humidity can increase, even if the actual amount of water vapor remains constant. If the air cools enough, it can reach the dew point, at which point the relative humidity is 100%, and the water vapor can condense into fog or clouds.