Water that is absolutely pure and free from any impurity is known as

Concept: Distillation is a physical purification process — water is boiled into vapour and then condensed back into liquid, leaving every dissolved mineral, salt, and non-volatile impurity behind in the original vessel. The condensed water is therefore the closest a common water source comes to pure H2O.

Application: Among the four sources here, only distillation actively separates water molecules from everything dissolved in them. Mineral water and spring water are drawn straight from the ground and carry whatever calcium, magnesium, and other minerals the rock and soil dissolved into them on the way up. Boiling water only raises its temperature enough to kill microorganisms — it does nothing to remove dissolved salts or minerals, so a boiled sample still carries the same impurities it started with. Distilled water is therefore the one option that is genuinely free of dissolved impurities.

Cross-check: Checking each source by what it actually contains confirms this:

• Mineral water carries dissolved minerals such as calcium and magnesium bicarbonates by definition; that mineral content is exactly what makes it "mineral" water.

• Spring water emerges from an underground aquifer after passing through rock strata, picking up dissolved minerals along the way.

• Boiled water: heating kills microbes but leaves every dissolved mineral and non-volatile impurity in the water unchanged.

So the water that is absolutely pure and free from any impurity is distilled water.