The purification method based on the following physical transformation is :

In some purification techniques, the solid sample is first converted directly into its vapour phase and then condensed back to the solid state, leaving behind the non-volatile impurities. This direct solid ⇌ vapour ⇌ solid change, without passing through the liquid phase, is called sublimation.

Only compounds that possess high vapour pressure at temperatures well below their melting points show this behaviour. Typical examples are naphthalene, camphor, iodine, benzoic acid, etc. Non-sublimable impurities (sand, salts, metal oxides) remain as residue.

Hence, the purification method that utilises the physical transformation “solid → vapour → solid” is sublimation.

Option C which is: Sublimation