In the horax bead test of ${\mathrm{Co}}^{2+}$, the blue colour of bead is due to the formation of:

In the borax bead test of ${\mathrm{Co}}^{2+}$, the blue colour of the bead is due to the formation of .

 The addition of cobalt salt causes the dark hue of the borax bead. In oxidising and reducing components, copper salt produces sky blue and red colours.

 The cobalt metaborate $\left(\mathrm{Co}{\left({\mathrm{BO}}_2\right)}_2\right)$ it gives the ${\mathrm{Co}}^{2+}$ test its blue colour.

 Hence, option C is correct.