In the extraction of nickel by Mond's process, the metal is obtained by :

In the extraction of nickel by Mond's process, the metal is obtained by thermal decomposition.

Nickel may be extracted and purified using the Mond process. Using this method, nickel oxides are transformed into pure nickel. The fact that carbon monoxide easily forms a reversible complex with nickel to produce nickel carbonyl is exploited in this method. The steps in the process are:

First, excessive carbon monoxide and impure nickel react at ${60}^{\circ }\mathrm{C}-{80}^{\circ }\mathrm{C}$ to create the gas nickel carbonyl, leaving the impurities as solids at ${180}^{\circ }\mathrm{C}$.

The reaction is as follows,

$\text{ Impure nickel }+\mathrm{CO}\stackrel{{60}^{\circ }\mathrm{C}-{80}^{\circ }\mathrm{C}}{\to }\mathrm{Ni}(\mathrm{CO}{)}_4\stackrel{{180}^{\circ }\mathrm{C}}{\to }\mathrm{Ni}+4\mathrm{CO}$

After that, 180°C is applied to the combination of extra carbon monoxide and nickel carbonyl. Nickel is produced when nickel tetracarbonyl breaks down under heat. Thus, the metal is produced by heat breakdown.

Hence, option B is correct.