**Core Concept**
The hardness of water is a measure of the amount of **calcium** and **magnesium** ions present in it. It is expressed in terms of **milliequivalents per liter (m Eq/L)**, which represents the concentration of these ions.

**Why the Correct Answer is Right**
One milliequivalent per liter is equivalent to one **millimole** of a monovalent ion or half a millimole of a divalent ion, such as **calcium** or **magnesium**, per liter of water.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately represent the equivalent of one milliequivalent per liter.
**Option B:** This option is also incorrect as it does not correctly relate to the ionic concentration in water hardness measurement.
**Option C:** Similarly, this option is wrong because it does not match the definition of water hardness in terms of milliequivalents per liter.

**Clinical Pearl / High-Yield Fact**
Water hardness is crucial in various medical and industrial applications, as it can affect the **formation of scale** in pipes and boilers, and also influence the **dose of detergents** needed for cleaning.

**Correct Answer:** D. 0.1 mmol of calcium or 0.2 mmol of magnesium per 1000 ml.