A reddish pink substance on heating gives off a vapour which condenses on the sides of the test tube and the substance turns blue. On cooling, water is added to the residue which turns into its original colour. The substance is :

Crystals of cobalt chloride are the material. It is a characteristic of ${\mathrm{CoCl}}_2$ 

Nickel and cobalt sulphide dissolves in aqua regia.

 A yellow precipitate of the cobalt complex known as potassium hexanitritocobaltate (III) is produced when the aqueous solution of the residue obtained after treatment with aquaregia is treated with a strong solution of potassium nitrite after neutralisation with ammonium hydroxide and the solution is acidified with dilute acetic acid.

$\mathrm{CoS}+{\mathrm{HNO}}_3+3\mathrm{HCl}⟶{\mathrm{CoCl}}_2+\mathrm{NOCl}+\mathrm{S}+2{\mathrm{H}}_2\mathrm{O}$

${\mathrm{CoCl}}_2+7{\mathrm{KNO}}_2+2{\mathrm{CH}}_3\mathrm{COOH}\to {\mathrm{K}}_3\left[\mathrm{Co}{\left({\mathrm{NO}}_2\right)}_6\right]+2\mathrm{KCl}+2{\mathrm{CH}}_3\mathrm{COOK}+\mathrm{NO}+{\mathrm{H}}_2\mathrm{O}$Hence, the option C is correct