**Core Concept:**
Hardness of water refers to the concentration of calcium and magnesium ions, which are responsible for the precipitation of soap and detergent residues in water. Hardness is measured in milliequivalents per liter (meq/L).

**Why the Correct Answer is Right:**
1 meq/L of water hardness corresponds to the presence of calcium (Ca++) and magnesium (Mg++) ions. These ions make the water hard because they interfere with the formation of soap and detergent lathers, leading to the deposition of these substances on surfaces, resulting in poor cleaning efficiency.

**Why Each Wrong Option is Incorrect:**
A. Option A is incorrect because it does not represent the concentration of calcium ions in meq/L, which is the given value for water hardness.
B. Option B is incorrect because it does not represent the concentration of magnesium ions in meq/L, which is also a component of water hardness.
C. Option D is incorrect as it does not relate to water hardness and does not correspond to any specific ions present in hard water.

**Clinical Pearl / High-Yield Fact:**
Understanding water hardness is essential for healthcare professionals, particularly in clinical practice, as it can lead to skin irritation, hardened pipes, and the formation of kidney stones due to the increased calcium and magnesium load.

**Correct Answer:**
**Correct Answer: D. 1 meq/L**

1 meq/L of hardness in water is equivalent to the presence of calcium (Ca++) and magnesium (Mg+) ions in solution. Hard water can cause skin irritation, increased risk of kidney stones, and damage to plumbing systems due to the precipitation of calcium and magnesium salts.