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