Evaporation is the process in which a liquid turns into a gas by gaining energy from its surroundings. It occurs when the molecules of a liquid gain enough energy to break free from the liquid's surface and enter the gas phase. Evaporation is a natural process that happens every day and has several practical applications. In the given options, evaporation can be helpful in all the cases:

a) Cooling water in an earthen pot:
- When water is stored in an earthen pot, the pot's porous walls allow water to slowly seep through and evaporate from the surface.
- As the water molecules gain energy from the surroundings and evaporate, they take away some heat, resulting in a cooling effect.
- This process of evaporation helps in keeping the water in the pot cool, making it suitable for drinking, especially in hot climates.

b) Obtaining salt from sea water:
- Evaporation is a crucial step in the process of obtaining salt from sea water.
- Sea water is collected in large shallow ponds or basins and left exposed to the sun and wind.
- As the water evaporates, it leaves behind the dissolved salt, which can then be harvested.
- This method is used in salt production industries around the world and is an effective way to obtain salt from sea water.

c) Obtaining powdered milk from milk:
- Evaporation is used in the production of powdered milk from regular milk.
- Milk is heated in large evaporators, which rapidly remove the water content from the milk.
- As the water evaporates, the remaining milk solids are left behind, which are then dried and powdered.
- This process helps in extending the shelf life of milk and makes it more convenient for storage and transportation.

d) All of these:
- Each of the given cases demonstrates the practical applications of evaporation and how it can be helpful in various situations.
- Whether it is cooling water in an earthen pot, obtaining salt from sea water, or producing powdered milk, evaporation plays a significant role in achieving the desired results.

In conclusion, evaporation can be beneficial in all the given cases. It helps in cooling water, obtaining salt from sea water, and producing powdered milk. Understanding the process of evaporation and its applications allows us to utilize its benefits in various aspects of our daily lives.