**Core Concept**
The World Health Organization (WHO) has established guidelines for the classification of water hardness based on its mineral content, specifically calcium and magnesium levels. Water hardness is measured in milligrams per liter (mg/L) and is categorized into four levels: soft, moderately hard, hard, and very hard.

**Why the Correct Answer is Right**
Water with a hardness level of 50-150 mg/L contains moderate levels of calcium and magnesium ions. This concentration of minerals is sufficient to provide beneficial effects on human health, such as improved bone density and cardiovascular function, without causing any adverse effects like scaling or corrosion of pipes. The moderately hard water category is characterized by a balance between the beneficial effects of minerals and the potential risks associated with excessive mineral content.

**Why Each Wrong Option is Incorrect**
**Option A:** Soft water typically has a hardness level of less than 50 mg/L, making it unsuitable for human consumption due to its inability to provide beneficial minerals.
**Option C:** There is no such classification as "Flard" water; it is likely a distractor intended to confuse the test-taker.
**Option D:** Very hard water has a hardness level exceeding 300 mg/L, which can lead to scaling and corrosion issues in pipes and appliances.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the optimal water hardness level for human consumption is between 50-150 mg/L, as this range provides a balance between the benefits of minerals and the risks associated with excessive mineral content.

**✓ Correct Answer: B. Moderately hard**