The correct option is A HCl gas through a saturated salt solution
Preparation of pure NaCl:
The crude salt is dissolved in minimum amount of water to make a saturated salt solution. Then, it is filtered to remove the insoluble impurities. The filtrate is then saturated by passing HCl through it, which gives the pure salt of NaCl by common ion effect. The salts of Ca and Mg remains in the solution due to their higher solubility.
Hence, the correct option is (a).