Permanent hardness of water is removed by:

1. Adding calcium sulphate
2. Boiling with potassium chloride
3. Boiling
4. Adding sodium carbonate

Adding sodium carbonate

Reason — Permanent hardness of water can be removed by addition of washing soda [sodium carbonate]:

Permanent Hard Water:

$\underset{\text{Washing soda}}{\text{Na}$2\text{CO}3} + \underset{\text{Calcium sulphate}}{\text{CaSO}4} \longrightarrow \underset{\text{Calcium carbonate}}{\text{CaCO}3↓} + \text{Na}2\text{SO}4

Calcium sulphate itself causes permanent hardness, so adding it will worsen the problem. Potassium chloride does not remove hardness. It has no effect on calcium or magnesium salts causing hardness. Boiling removes only temporary hardness (due to bicarbonates), not permanent hardness.