The correct answer is (d), rain water.
Rain water is the purest form of water because it has not come into contact with any other substances. It falls from the sky and is collected in rain barrels or other containers. Rain water is often used for drinking, bathing, and watering plants.
Well water is water that comes from underground aquifers. It is often filtered through rocks and soil, which can remove some impurities. However, well water can also contain bacteria and other contaminants.
Spring water is water that flows out of the ground from a spring. It is often considered to be purer than well water because it has been filtered through the earth. However, spring water can also contain bacteria and other contaminants.
Surface water is water that is found on the surface of the Earth, such as in lakes, rivers, and oceans. It is often contaminated with pollutants from human activity, such as sewage, industrial waste, and agricultural runoff.
Therefore, rain water is the purest form of water because it has not come into contact with any other substances.