Introduction:
A desert cooler is a device that provides comfort by cooling the air through the process of evaporation of water. It is designed to work more effectively in certain weather conditions. Among the given options, the desert cooler works most effectively in hot and humid weather.

Explanation:

Hot and Humid Weather:
In hot and humid weather conditions, the desert cooler works more effectively due to the following reasons:

1. Evaporation: In hot and humid weather, the air already contains a significant amount of moisture. When water is added to the cooler, it evaporates at a slower rate compared to dry weather. This slower evaporation process helps to cool the air more effectively.

2. Less Evaporative Cooling: In humid weather, the air is already saturated with moisture. This means that the air cannot hold much more water vapor. As a result, the desert cooler's evaporation process is less efficient in such conditions. However, even with reduced evaporation, the cooling effect is still noticeable and can provide some comfort.

Other Weather Conditions:

1. Rainy Day: On a rainy day, the air is already humid due to the presence of rainwater. The desert cooler's evaporation process would be less effective as the air is already saturated with moisture. Additionally, the rainwater would make the air cooler naturally, reducing the need for additional cooling.

2. Hot and Dry Weather: In hot and dry weather, the desert cooler works relatively effectively. Dry air has a lower moisture content, allowing for faster evaporation of water. The cooling effect is enhanced as the water evaporates quickly, leading to a more noticeable drop in temperature.

3. Hot and Sunny Day: A hot and sunny day would also be suitable for the desert cooler to work effectively. The presence of sunlight increases the rate of evaporation, making the cooling process more efficient. The combination of heat and sunlight helps to enhance the cooling effect of the desert cooler.

Conclusion:
The desert cooler works most effectively in hot and humid weather conditions. While it may not be as efficient in humid weather compared to hot and dry conditions, it still provides a noticeable cooling effect.