B)rain because the soap can give more lather in pure water.The rain water is the best source of pure water among the given options.
The answer is B) Rain water  ; when can get (almost)  pure form of water from the rain water; since Tap , well and Handpump water contains Calcium(Ca²⁺) and Magnesium(Mg²⁺) ions These binds with the soap molecule forming insoluble soap dirt (lime soap) , which results in wastage and decrease in action of soap. literally, Rain water due to absence of Ca and Mg ions is termed Soft water.
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