The presence of humidity in the atmosphere can be ensured by observing water drops on the outer surface of a steel glass filled with [B] hot water. When hot water is poured into the steel glass, it warms the air inside, causing moisture in the air to evaporate. As the warm, moist air comes into contact with the cooler surface of the steel glass, it cools down, leading to condensation of water vapor on the outer surface of the glass. This condensation forms droplets, indicating the presence of humidity in the surrounding air. Hot water provides the necessary warmth to generate sufficient moisture in the air for condensation to occur, making it an effective method for observing humidity. Therefore, option [B] is the correct choice for ensuring the presence of humidity.