Explanation:

- Boiling: This method is effective for temporary hardness but does not remove permanent hardness, which is caused by dissolved calcium and magnesium salts.

- Ion-exchange: This is a highly effective method for removing permanent hardness from water. It works by exchanging calcium and magnesium ions with sodium or potassium ions, thus softening the water.

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- Lime treatment: It is effective primarily for removing temporary hardness. It works by precipitating calcium carbonate and magnesium hydroxide but is not commonly used for permanent hardness.

- Bubbling sulphur dioxide gas: This method is not usually used for removing permanent hardness from water. It's not effective for this purpose.

Thus, the ion-exchange method is indeed the correct option for removing permanent hardness.