Which of the following is considered to be the purest form of natural water (distilled water) free from impurities?

Rain water

The correct answer is option 4.  Rainwater.

Rainwater is often considered one of the purest forms of natural water as it starts as condensed water vapor in the atmosphere and falls as precipitation. During this process, it bypasses many of the impurities and contaminants found in other water sources like well water, spring water, or river water. However, it can pick up pollutants as it falls through the atmosphere, so the actual purity of rainwater can vary depending on factors like air pollution and local environmental conditions. Nonetheless, properly collected and stored rainwater can be relatively pure, akin to distilled water.