Which of the following is suitable for water purification?

Water purification and softening by the ion-exchange method work through materials whose crystal structure loosely binds exchangeable ions — most often Na+ or Ca2+ — inside a cage-like porous framework. When such a material contacts hard or impure water, its own loosely held ions pass into solution while the water's dissolved impurity ions (such as Ca2+/Mg2+, which cause hardness) are captured in their place. A substance's suitability for this kind of purification therefore depends on whether it has this open, ion-exchanging framework, not merely on being chemically inert or mechanically hard.

Among the given options, zeolites are hydrated aluminosilicate minerals with exactly this kind of porous, cage-like crystalline framework, loosely holding Na+ and Ca2+ ions. Because these ions readily exchange with other ions present in a solution, zeolites function as natural ion-exchange materials — the property widely used to soften and purify water by replacing dissolved Ca2+/Mg2+ ions with Na+ ions. This is exactly the chemistry exploited in zeolite-based water softeners and purification units, both in laboratories and municipal water treatment.

• Silicones are synthetic silicon-oxygen polymers valued for sealing, lubricating, and water-repelling behaviour — a property profile built around inertness and hydrophobicity, not ion exchange.

• Asbestos is a naturally occurring fibrous silicate mineral valued historically for heat resistance and fire insulation — its fibrous structure offers no ion-exchange sites.

• Quartz is a hard, crystalline form of silicon dioxide (SiO2) valued for its optical clarity and piezoelectric behaviour in glass, electronics, and precision instruments — its rigid lattice does not provide the loosely bound, exchangeable ions that purification by ion exchange requires.

The ion-exchange chemistry described above is unique to zeolites among the given options, making zeolites the correct choice for water purification.