Which of the following is used for the preparation of colloids?

A colloid is a heterogeneous system in which one substance (the dispersed phase) is distributed as very fine particles throughout another substance (the dispersion medium). The particles in a colloidal solution have sizes in the range:

$$1 \text{ nm} < \text{particle size} < 1000 \text{ nm}$$

That is, $$10^{-9}$$ m to $$10^{-6}$$ m (or equivalently $$10^{-7}$$ cm to $$10^{-4}$$ cm).

Now let us examine each option:

Option A: Ostwald Process — This is an industrial process for manufacturing nitric acid ($$HNO_3$$) from ammonia. It involves catalytic oxidation of $$NH_3$$ over a platinum-rhodium catalyst. This is not related to colloid preparation.

Option B: Van Arkel Method — This is a method for purifying metals (especially Ti, Zr, Hf) by converting the metal to a volatile iodide and then decomposing it on a hot filament. This is a metallurgical purification process, not a colloid preparation method.

Option C: Bredig's Arc Method — In this method, an electric arc is struck between two electrodes of the metal (e.g., gold, silver, platinum) immersed in the dispersion medium (usually cold water with a small amount of stabilising electrolyte like KOH). The intense heat of the arc vaporises the metal, and the vapour condenses into particles of colloidal size in the surrounding cold liquid. This is a well-known electrical disintegration method for preparing metal sols (colloidal solutions of metals).

Option D: Mond Process — This is used for purifying nickel. Impure nickel is treated with carbon monoxide to form volatile nickel carbonyl $$Ni(CO)_4$$, which is then decomposed to give pure nickel. This is not a colloid preparation method.

Therefore, among the given options, Bredig's Arc Method is the one used for the preparation of colloids.

The correct answer is Option C.