Which process is used to remove permanent hardness?

Permutit process is used to remove permanent hardness. Because boiling cannot dissolve persistent hardness.

Permutit process is used for filtering hard water containing calcium (II) ions using zeolite sand. Permutit is a zeolite from an artificial source. It is an orthosilicate of sodium aluminum compound with a chemical formula ${\mathrm{Na}}_2{\mathrm{Al}}_2{\mathrm{Si}}_2{\mathrm{O}}_8\cdot {\mathrm{xH}}_2\mathrm{O}$.

But the washing soda, Ion exchange resin, and Gan's Permutit methods can all be used to eliminate the water's permanent hardness.

Hence, the correct option is D.