The permanent hardness of water is caused by the presence of certain minerals, primarily sulfates and carbonates. Option 1 states that the hardness is due to the presence of sulfates of sodium and potassium. While sulfates can contribute to hardness, the presence of sodium and potassium sulfates alone would not cause permanent hardness. Therefore, option 1 is incorrect.
Option 2 states that the hardness is due to the presence of sulfates of magnesium and calcium. This is the correct answer. Magnesium and calcium sulfates are common minerals found in water sources and are known to cause permanent hardness.
Option 3 suggests that the hardness is due to the presence of carbonates of sodium and magnesium. Carbonates can contribute to temporary hardness, but they do not cause permanent hardness. Option 3 is incorrect.
Option 4 states that the hardness is due to the presence of bicarbonates of magnesium and calcium. Bicarbonates can also contribute to temporary hardness, but they are not responsible for permanent hardness. Therefore, option 4 is incorrect.
In conclusion, the correct answer is option 2, as it correctly identifies the sulfates of magnesium and calcium as the cause of permanent hardness in water.