**Core Concept**
Soft water is defined as water that has a low concentration of dissolved minerals, particularly calcium and magnesium ions. These ions contribute to the water's hardness, which is typically measured in terms of the water's ability to precipitate soap into a sticky scum.

**Why the Correct Answer is Right**
Water hardness is determined by the concentration of calcium and magnesium ions. Soft water has a low concentration of these ions, typically less than 60 mg/L (milligrams per liter) as calcium carbonate (CaCO3). This low concentration of dissolved minerals makes soft water ideal for various industrial and household applications, such as textile manufacturing and cleaning.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the exact concentration of calcium and magnesium ions that define soft water.
**Option B:** This option is incorrect because it is too vague and does not provide a specific concentration of calcium and magnesium ions.
**Option C:** This option is incorrect because it is too high and would actually define water as hard.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the definition of soft water can vary slightly depending on the country or region. However, in general, soft water is considered to have a hardness level of less than 60 mg/L as CaCO3.

**Correct Answer: D. Less than 60 mg/L**