**Core Concept:** The WHO (World Health Organization) recommends a suitable range for drinking water hardness to ensure safe consumption and maintain good health. Hard water refers to the presence of calcium and magnesium ions, which can affect water quality and cause potential health issues if consumed in excessive amounts.

**Why the Correct Answer is Right:** The correct answer is **D** (180-300 mg/L), as per the WHO guidelines. Drinking water with a hardness level between 180-300 mg/L is considered suitable for human consumption, as it does not pose significant health risks.

**Why Each Wrong Option is Incorrect:**

A. 0-180 mg/L (Option A): This range is too low, as it may not effectively remove minerals and soap residues, leading to soap scum and reduced effectiveness.

B. 301-500 mg/L (Option B): Water in this range is considered excessively hard, which can cause health issues like kidney stones, kidney damage, and increased risk of cardiovascular diseases due to elevated calcium and magnesium levels.

C. >500 mg/L (Option C): Water with a hardness level greater than 500 mg/L is extremely hard, posing severe health risks, as mentioned above.

**Clinical Pearl / High-Yield Fact:** Hard water can lead to health issues like kidney stones, kidney damage, and increased cardiovascular risks due to elevated calcium and magnesium levels. Maintaining the correct hardness level (180-300 mg/L) ensures safe drinking water for human consumption and helps prevent these potential health problems.