**Core Concept**
Water hardness is a measure of the concentration of calcium and magnesium ions in water, expressed in milliequivalents per liter (mEq/L). This measurement is crucial in determining the suitability of water for various uses, including drinking, industrial processes, and agricultural purposes.

**Why the Correct Answer is Right**
The correct answer is in milligrams per liter (mg/L), which is equivalent to milliequivalents per liter (mEq/L) for calcium and magnesium ions. This is because the equivalent weight of calcium and magnesium ions is 20 mg/equivalent, and their concentration in water is typically expressed in milligrams per liter. The milliequivalent weight of calcium and magnesium ions is 1/1000 of their equivalent weight, which is why 1 mEq/L is equivalent to 20 mg/L.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not directly related to the measurement of water hardness.
**Option B:** This option is incorrect because it is a unit of measurement for pH, not water hardness.
**Option C:** This option is incorrect because it is a unit of measurement for osmolarity, not water hardness.

**Clinical Pearl / High-Yield Fact**
For exam purposes, remember that water hardness is an important factor in determining the suitability of water for use in various industries, including textile and paper manufacturing, as well as in agriculture.

**Correct Answer: C. milligrams per liter (mg/L)**